We paid our photographer 100% before the wedding.
Something to look out for; Make it absolutely non-negotiable that one of the following happens:
1: All photographs are considered "work for hire", and you will be the copyright holder. You may optionally give the photographer a license to use them for promotion.
-OR-
2: If the photographer holds the copyright, you have a license to use all photographs as you see fit.
You must also make sure you will be provided with all the "digital negatives" (high quality JPG or RAW) images straight from the photographer's camera.