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.