How can we ask people not to take pictures during the ceremony?
The best way to tell someone that they can't do something is to tell them when they can. Schedule the most photogenic part of the ceremony -- the kiss -- at the very end, and clue your guests in to the coming photo op.
Use a double-pronged approach to let people know: First, insert a line in the program, below the processional, that says "No photographs during the ceremony, please," or some such. Second, prime your ushers to tell people as they're seated that you are requesting no photographs until the very end of the ceremony.
Once it's time for the kiss, pause at the top of the aisle to give people a chance to take a picture before you walk out.