Before I was a florist, I couldn't tell a carnation from a rose, and definitely didn't know what lilies looked like. I wouldn't know where to begin come my wedding day! My advice, find someone whose work you love, and trust them. My biggest tip -- let them do what they do best! Here are some questions you may not have thought of, but are good to ask:
1. Are my flowers in season? Seems so obvious, right? Unless you are using year-round roses, tulips or hydrangea, you may not be thrilled to find out that your absolute favorite flower, the one you've had your heart set on, isn't available. Peonies in August? Ranunculus in September? Probably not happening. Your florist should provide substitutions for similar flowers, so that those peonies become garden roses.
2. Do you charge to breakdown? Depending on your venue, and your preference, you may want to keep the centerpieces to give away to guests. This is a great way to use your flowers as party favors, especially if you do a bunch of smaller vases versus one large centerpiece. The florist may charge you for keeping the vases (it should only be 10% more or so), but should deduct their normal rate for setup and breakdown if they don't have to come back to the venue that night.
3. What's your contract policy? Make sure you sign a contract with your florist. You want to be on the same page about the details, such as the right to use substitutions, rentals, and deposit amounts. It is normal to put half of the amount down for a deposit, and the rest in full a month before the wedding.
4. How many consultations to do I get? There's probably not a limit to how many times you can call them or email them, but you will want to ask about samples. Every florist has their own policy on providing sample bouquets and centerpieces -- and how much they charge for each one. Usually, if you sign a contract, with the florist they will provide a sample gratis, but you will want to ask them their policy beforehand.
5. When will we do a walk-through? This goes along with the above point, but it's often overlooked. I think it's important for the florist to do a walk-through with the bride so that they can really see the setup of the place, which will definitely influence their suggestions and designs. If the venue is out-of-town then of course it's unrealistic to ask, but going through the venue photos together will help immensely so that you're both on the same page.
6. Will you be the one designing on my day? The first time I had a bride ask me that, I must've had a perplexed look on my face. "Of course I will be designing on your day!" I replied. She then explained how worried she was that she would go with a florist and the designer she met with would no longer be working there on her day. This is a valid concern, and worth asking. You want to make sure to avoid the horror stories I've heard, from the owner or main designer not being there, to them messing up the colors, or not providing enough boutonnieres. This all comes back to trusting who you hire -- read reviews, ask around, and most of all, trust your gut. Your designer will know the setup time, how much time is allotted, and you may also want to ask how many people they plan to bring. A small wedding requires two, and large can be five and up. In the end, all flowers are beautiful, so once you have the designs down, relax, and enjoy the journey.