Avoiding these common mistakes can give that dollar in your pocket a little more company.

By Candace Braun Davison

You're Not Buying A Shiny New Phone
Whether you buy it now or you buy it later, getting a new phone means paying for it. But keeping your current phone after your two-year agreement is up means you might be making payments on it long after you've covered the full price of it. Many service agreements charge a subsidized rate for a phone upfront -- about $200 -- plus a monthly payment fee that's included in your bill. When the 24 months are up, that fee remains part of your monthly statement, explains Consumer Reports. Until recently, you haven't had many other options. But, as of March 2013, T-Mobile allows you to use your current phone on a lower-rate, "no contract" month-to-month plan. Unlimited minutes, text messages and 2GB of high-speed data costs $60 per month. A comparable two-year contract plan elsewhere can cost $100 a month, which means that sticking with your current phone -- and staying in a contract -- could cost you up to $40 more per month.

If you really want a new phone, CNET found that opting for a no-contract plan and buying the phone at full price may still be cheaper than getting the same phone at a subsidized rate with a two-year agreement. An iPhone 5 and two years' service costs $2,020 at T-Mobile, but it can cost $2,600 elsewhere, the site reports.

You're Buying in Bulk
It seems like a good idea: Get more of what you use for less. But bulk buying is one of six leading causes of food waste, which can cost a family of four an estimated $1,350 to $2,275 each year, reports the Natural Resources Defense Council. When it comes to items without an expiration date, the problem often isn't using too little -- it's blowing through too much, too fast: Turns out 53 percent of people don't use the recommended dose of laundry detergent -- guessing or filling the cap to the top -- found a 2010 study by Method Products, Inc. Some caps have fill lines halfway, some are two-thirds, but it's almost never near the top, says Merany Eldridge, spokeswoman for Method. (To combat this, Method's detergents use a pump nozzle for a more precise measurement, and an increasing number of companies offer single-serving tablets, like Tide Pods.)

You Refuse to Be Upsold at an Oil Change
Maybe you have a story like this: A friend brought her car in for a $30 oil change and wound up paying $3,000 for unrelated "urgent" repairs that turned out not to be so urgent. Still, the top things a mechanic might suggest are fluid changes, tire rotations and new brake pads -- which you may genuinely need, says Austin Davis, author of What Your Car Mechanic Doesn't Want You to Know and founder of MyHonestMechanic.com. By letting the mechanic, who's already got a good look at your engine, check out key areas, you can save yourself an extra trip to a dealer or a surprise problem on your next road trip. Davis recommends keeping these facts in mind: Transmission fluid should be changed every 50,000 miles, the power steering fluid once in the car's lifetime (roughly 100,000 miles), and your tires should be rotated every 9,000-10,000 miles. "The front brakes need to be replaced about twice as often as the back ones, because that's where the stopping power is," he explains. If a mechanic urges you to change them, ask how thick the brake pad is compared to its metal backing -- if the pad is thinner than the backing, it's time to replace them.

You Turned Down The Warranty
Warranties don't always make sense -- like paying $3 for a two-year service plan on DVDs that cost $5, something we recently came across -- but for expensive items you don't have the funds to replace, the extra reassurance could be worth it, says Manisha Thakor, CEO of MoneyZen Wealth Management LLC. About one in five people will have their iPad damaged within the first two years of ownership, reports SquareTrade, which sells two-year warranties for $100. (In our experience, frequent travelers and those who are a little on the klutzy side are well represented in this group.) Like many companies, Apple's standard warranty doesn't cover accidental damage, so any cracked screens, scratches or broken ports are up to you to replace. But the company does offer AppleCare+, which will cover up to two "oops" moments for $100 for two years, plus a possible $49 service fee. Wal-Mart also offers a two-year accident-protection plan for $69 for tablets that cost $500-$700.

You Bought A Pass To Save On Tolls
Getting an electronic tolls pass -- such as an E-ZPass or SunPass -- to use with your rental car can save you a few dollars on select tolls compared to paying with cash (up to $4.75 when crossing the George Washington Bridge or Lincoln Tunnel in New York), but if you're not hitting the tolls every day you rent the car, it may not be worth it. Some car rental companies charge up to $5 per day for use of the pass -- even on days when you never pass a toll plaza. Plus, some advertised discounts come with restrictions. For example, you can cross the Golden Gate Bridge into San Francisco for half price with a FasTrak pass, but to get the full 50 percent off, you must have three or more people in the car and be crossing during weekday rush hour. Otherwise, using FasTrak shaves $1 off the $6 toll. USA Today offers a roundup of many of the major rental car companies' policies so you can check yours at a glance before booking.

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Money Lessons You Don't Need To Follow
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  • Old Lesson: Pay Off Your Mortgage ASAP

    Here's what I've been taught about money: It's not good to be in debt, and therefore it is good to pay off debts as quickly as possible, including a mortgage. Turns out, this conventional wisdom is a holdover from a different time. Carmen Wong Ulrich, the president and co-founder of <a href="http://www.altawealthmanagement.com/" target="_blank">ALTA Wealth Management</a>, told me: "People think that it's a good idea to pre-pay a mortgage and/or pay a mortgage off ASAP." What's wrong with that? "This advice comes from a previous couple of generations who had horrible interest rates on their mortgages," says Ulrich. "In the 1980s, the average mortgage interest rate was in the teens. Imagine: My dad, who had great credit, was paying 17 percent on his mortgage. That's expensive. Today, mortgage rates are at incredible lows -- you should be in no rush."

  • Old Lesson: Bank With The Local Guy

    Supporting local businesses is great, but you don't need to follow your parents' footsteps in keeping their life savings at First Local Urbantown Bank with No Branches Anywhere Else. There are banks that exist entirely online, and experts say they are not only safe but can save you a lot of money in fees. Kiplinger.com points out that because <a href="http://www.kiplinger.com/slideshow/saving/T005-S001-best-deals-in-online-banking-2012/index.html">online-only banks don't have the same overhead costs as traditional banks</a>, they can offer high interest rates on checking and savings accounts. Kiplinger rated the best deals in online banking <a href="http://www.kiplinger.com/slideshow/saving/T005-S001-best-deals-in-online-banking-2012/index.html" target="_blank">here</a>.

  • Old Lesson: All Student Loans Are 'Good Debt'

    Personally, I would never tell anyone to not go to college if they have the option, from a self-betterment/learn about the world/read lots of books angle. But when it comes to cold hard financial facts, an expensive four-year college education might not actually make the most economic sense, particularly if it's going to mean embarking on grown-up life with an oppressive load of debt. As <a href="http://www.richdad.com/Home.aspx" target="_blank">Robert Kiyosaki</a>, the author of the bestselling <i>Rich Dad, Poor Dad</i> series, told me, in today's world college isn't always a financially sound investment. Kiyosaki said, "It's tough to know -- on the front end, when a student is taking on school loan debt -- what the ROI [return on investment] will be. Will the degree he or she earns at a cost of $100,000 in school load debt land them a $200,000 a year job? Or a $50,000 job? For many students, school loan debt is an albatross around their necks as they begin their adult lives." (Then again, if they go to a liberal arts school, they'll get <a href="http://en.wikipedia.org/wiki/The_Rime_of_the_Ancient_Mariner">the albatross reference</a>.) So the idea is, from a financial standpoint, young people may want to think carefully about their future career prospects before taking out loan for a super-pricey education.

  • Old Lesson: Saving For Your Kids' College Should Be Priority Number 1

    So if we don't want to send our children into student-loan debt land, we should be sure to save as much as we can in case they do want to go to college, right? The personal finance journalist and bestselling author of <i><a href="http://www.amazon.com/Get-Financial-Life-Personal-Twenties/dp/0743264363" target="_blank">Get a Financial Life</a></i>, Beth Kobliner, says this is another big "nope." "<a href="http://www.bethkobliner.com/" target="_blank">It's definitely smarter for parents to max out their own retirement plan</a>," says Kobliner. "Particularly if their company will match their contribution -- something they won't find in a college savings plan. And while kids can borrow for their college education, parents can't borrow for retirement, which means they may have to depend on their kids (who may already be struggling to make ends meet for <i>their</i> kids) to support them down the road." And that's not all, Kobliner says. "A new study released earlier this month shows that the more money parents contribute to their kid's college education, the lower their child's GPA. The gist is that kids are just not as motivated to achieve, so they do well enough to graduate but don't push themselves to be their best." To which everyone who had a college roommate with a major in Modes of Recreational Drug Use in Party Settings says, "Yep."

  • Old Lesson: When You Have Some Extra Money, It's Time For A New Couch

    It sounds like the right advice: Use all your money to buy actual things that actually exist (and in a pinch could be sold on eBay). In the past 10 years, there's been a lot of discussion about the money-happiness dynamic, and it turns out that while a new couch may make your house better, it might not be the best way to make your life better. Elizabeth Dunn, Ph.D., University of Virginia, co-author of <i><a href=" http://www.amazon.com/Happy-Money-Science-Smarter-Spending/dp/1451665067" target="_blank">Happy Money: The Science of Smarter Spending</a>,</i> has done extensive research on how money can "buy happiness." Her advice? "Seek experiences: Vacations and time with friends provides more happiness than simply buying more clothes or widgets for the house. The money spent is more meaningful and memorable." Your parents may think it sounds impractical, but this isn't just about a bank account -- it's about a life. And it's not theirs (anymore), it's yours.

  • One Old Lesson You SHOULD Follow: Be An Investor, Not A Collector

    With all the personal finance advice we have at our fingertips (and on our screens), some of us tend to get excited about hot new stocks and over-diversify, collecting lots of investments that duplicate one another. But that can be <a href="http://store.behaviorgap.com/collector-not-an-investor/" target="_blank">detrimental to your financial future, says Carl Richards, a certified financial planner</a>, on his site <a href="http://www.behaviorgap.com/" target="_blank">BehaviorGap.com</a>. The problem is, as he writes, "there's no cohesive investing strategy at work... This over-diversification can lead to inefficiencies, including unnecessary taxes, internal expenses, transaction costs and, maybe most importantly, the impact on your life in the form of the time you have to spend thinking about all those lines on your monthly statement." So while it feels very modern and high-tech to dive into a bunch of individual funds (and manage them from your phone), here might be one place where an old-fashioned approach -- a few well-chosen funds, overseen by a financial planner -- can serve you well.