A - Have a Concept... of what you want. Then begin your search through recommendations, the web, and a phone interview. You may also wish to check with the Better Business Bureau before proceeding. Check availability.
B - Check out about 3 musicians to find the one that best suits you. You have to know that whether you are hiring an amateur or a professional that you will be happy with the performance. If you are dealing with a company, talk to the musicians themselves.
You can ask to hear a recording, see a videotape, or perhaps preview the person/group in action at another event.
If this is for your reception dance, as you are interviewing the musician(s), get a sense of their personality. Are they confident and professional? Do you think they could respond to a group and motivate them? How frequent and long are the breaks?
C - Check the references. Ask about things such as punctuality, politeness, and dress code as well as how well they liked the final product. Ask what they liked, if there were any problems, and if they would recommend this caterer.
D - Sign the contract. Make sure it includes the location, the arrival and exit times, the final cost with hidden charges (such as traveling) and HST/GST. Back-up provisions, length and frequency of breaks should be included. Overtime and cancellation policies should also be in writing, as should the fact that the musician or group carries insurance.