We've all heard that people get jobs based on who they know, but what if you aren't lucky enough to be acquainted with someone in the field you're pursuing?
Your first step is to tell everyone you come across -- including family, friends, neighbors, the people in the doctor's office, fellow churchgoers, etc. -- about what you'd like to do. Ask the people you talk to if they know anyone who might be willing to grant you an informational interview. Informational interviews are typically 30 minute conversations in which you learn more about a contact's career trajectory. They are a terrific entry point to establishing a potentially valuable long-term relationship.
Other tried and true techniques include joining relevant third-party associations (American Marketing Association, etc.) and attending their events, and doing searches on popular social networks like LinkedIn and Facebook.
One of my favorite strategies involves targeting a handful of target companies where you're interested in working and taking proactive measures to establish personal connections with people inside those companies. Now, these people need not be HR managers. Rather, you should call around and look on the Net to find the names of people who work in your proposed department.
Once you've identified a contact, let's say his name is Mr. Golden Opportunity, find out as much about him as you can, and then craft a short, friendly e-mail introducing yourself and what you're looking for.
Even if Mr. Golden Opp is not the person who would ask you in for an interview, you've now made it inside the company. Mr. Golden Opp probably has the internal contacts to introduce you to the person who can hire you. Perhaps he will forward your information directly or will mention to his colleague that you'll be contacting her.
Here's the thing about networking: it won't do to simply sit around and wait for an opportunity to meet people who are in a position to help you. The most important networking interactions don't just happen -- they're made. Even if you speak to someone who can't assist you right away, don't lose the contact. Follow up at appropriate intervals so the person doesn't forget who you are. You never know when the relationship might come in handy in the future.