1. Do your research. I've said it before and I'll say it again: just because Uncle Bob takes great pictures of his dog doesn't mean he can take a good headshot. Same goes for wedding, boudoir, senior portraits, etc. Each style requires a particular set of skills and different types of equipment. Not all photographers can cross-shoot into every style. Don't assume they can. Do your research and find the right person for the job.
2. Choose a photographer who's work you love. Sounds easy and obvious, right? Yet so many times people expect the photographer to deliver images outside of their expertise. That is a very good way to end up disappointed! Things to look at include: lighting, locations, colors, angles, posing and editing. I, for example, like to photograph editorial style. I photograph clients the way I would photograph a model for their portfolio. I don't pose my clients too much. Instead, I help bring out their personality by making them laugh and putting them at ease and guiding them as we go. I then capture them being...well, them! I love laughs, big laughs. I love closed eyes and silly faces and beautiful moments among family and couples kissing. I want that goofiness that we all have inside to show. If you are looking for traditional portrait photography, with a pillar under one arm and a rose in the other, I am not the way to go.
3. Ask to see a full gallery. Most of us photographers put our absolute best work on our websites. Check to see if the photographer you are considering publishes a blog as well. Blogs are a great way to see more of what each client gets per shoot. On the website, there may only be one image from a portrait session, but on a blog entry, there would surely be more. If the photographer does not have a blog, ask to see a client gallery. That way you'll know exactly what to expect.
4. Know exactly what you're getting. Every photographer has a different business model. You may be paying for the session and be expected to buy the prints or "files" later. Or perhaps the high resolution images are included. Will you get a CD, USB Drive, or a gallery from which to download? Ask what the session includes, what the turn-around time is, if editing is included, etc.
5. Send samples. If you see pictures you love on Google or Pintrest, e-mail them to your photographer. Plan your session. If you have a certain vision or idea, let your photographer know. Otherwise, you can't be upset about not getting what nobody knew you wanted in the first place!
Hope this helps! Happy posing!
-V.