Get a written contract that includes all the details that you have discussed with the videographer. This will include locations, times and the specific events at the wedding you want to have videotaped or recorded.
A nice touch is to have an area set up at the reception where guests can come and record their well-wishes to the bride and groom. If you are planning on offering this to the guests, be sure to have the Master of Ceremonies make an announcement when it will start.