Consider the size of your guest list. Don't hire a 12 person band for a wedding with 25 people, consider something a little more intimate like a solo violinist or a harpist. But on the flip side, a soloist works for any number of people.
Get a contract. Don't just assume you know what the musician expects, and don't assume the musician knows what you expect.
Consider the theme of your wedding. Don't hire an electric guitarist for a traditional wedding, and don't hire a classical violinist if you want Jazz. Make sure the musician can play the style of music you want before you hire them.
Check with your venue. Make sure they allow outside musicians, some venues only allow certain types of music or will only let you hire their pianist.
Price and Overtime Rates. Expect to pay a deposit when you hire a musician, and just like other wedding contractors there are set fees for everything. If you need extra songs during the ceremony, there are fees for that, if you need a consultation there are fees for that. Also, make sure you understand overtime rates, just because your reception goes longer than expected doesn't mean that musicians won't expect extra money, or even that they will stay the whole time.
Check with your spouse-to-be. You would be surprised at the number of grooms that actually do care. At least ask if they have an opinion instead of assuming they don't.
Cancellation Policy. Make sure that this is in the contract. And don't expect to get a refund or not owe anything if you cancel on the musician the week before. As a musician, by that point we've already put hours of work into making your wedding perfect, just because you decide to go a different route doesn't mean you're off the hook. Also make sure what happens if the musician cancels on you!
Ask for references and a recording. Just because their website says they can play the guitar doesn't make them the perfect match. Make sure you will be pleased with the music they will play.
Make sure there's enough space. Even a soloist needs a good bit of space, a band needs even more. Keep in mind there's not just people but equipment as well (instruments, music stands, microphones, chairs, etc.)
Know your playlist BEFORE you sign the contract. Just because it's your favorite song doesn't mean the musician can play it.
Make sure every detail is covered. Directions, where to set up, where to put cases, emergency contact numbers. Leave no detail out so everything will go smoothly for the perfect, hassle-free, romantic wedding!