Questions to ask yourself:
1. Does his/her work appeal to me?
2. Would I want this work in my home?
3. Are her/his services in my price range?
4. What are my goals for this session? Will the photographer accommodate them?
5. Do I want to bring any props or changes of clothing to the shoot? Will that be okay with the photographer (and ask him/her).
1. Ask around. Do you know anyone who has used this person before? Were they happy?
2. Contact the photographer and ask questions. Though many provide detailed FAQ sections (I'm happy to say I do), you may just be new to custom photography and it could all be a bit overwhelming. A professional will be happy to spend the time it takes to quell your fears and answer those burning questions they may not have covered on their website (but you should definitely check their site first since, and I speak from experience, they did take the long to write out all those thoughtful answers ;^D ).
3. Make sure you know BEFORE you book an appointment exactly what you're getting. Fighting over services is never pretty, for either party, and as a photographer, I would truly rather spend my time shooting than arguing.
4. Pay your deposit ASAP to secure your date. You don't want to miss out because you didn't want to fork over cash you would have to pay anyway. A deposit lets the photographer know you're serious, and we like committed customers!
5. Plan ahead. I cannot tell you how many times I have seen people scramble, and it's always over clothing. The day of the shoot you want to be thinking about nothing but the excitement of capturing memories. Take your photographer's advice and follow their guide or verbal advice (I have a guide on my site because people are people and someone always gets overwhelmed about clothes).
6. Once you've made your choice, TRUST THE PHOTOGRAPHER! If you are nervous and/or uncomfortable, it will show. Believe in them, allow them to work freely, know that they have the talent and experience to do the job well. After all, you did choose them over all the others. Let them show you their best work!
I do hope this helps you. While I know I am not the photographer for everyone, I like to think I can photograph anyone. I'll let you be the judge ;^)