1) Get Your Negatives!
I can't stress this enough. Make absolutely *certain* that your photographer's fee includes your right to get and duplicate any and all photos the photographer takes! There should be an option to get all negatives on DVD, and this should be included in the photographer's fee. Prints might be an additional fee, but you should be able to do that yourself, where and when you choose.
2) View Entire Albums
Don't settle for a sampling of their best photos - make sure you see an entire wedding from start to finish. This gives you a far more realistic impression of what you can expect should you hire them for your wedding.
3) Meet Your Photographer
Photographers vary greatly in quality and experience. If you use an agency to hire a photographer, make sure you get that photographer whose work and personality you appreciate. Don't let an agency simply assign whoever is available, potentially switching the photographer you get (possibly for a less expensive, less experienced photographer) at their whim.
4) Like Your Photographer
You'll be surprised how much your photographer's personality matters on your wedding day. For that whole day, they will effectively be your best friend, staying nearby every bit as much as (if not more than) your closest family members - or they're doing something wrong.
5) Don't Get Hung Up on Fancy Technology
A poor craftsman blames their tools. Film, digital; Canon, Nikon; these things do not make a good photographer. Judge each by their abilities, not the toys they've acquired. Don't get me wrong - I'll be among the first to geek out about cameras, but don't dismiss someone simply because they don't use a technology you prefer; you could give a good photographer a disposable camera and still expect decent, if not remarkable results.