1. Get umbrellas. If you think there is a possibility of rain on your wedding day, be prepared with matching umbrellas. The larger the dome, the cuter. If the bride and groom have one of those cute domed clear umbrellas, even better. Go online and find some pictures of weddings photos in the rain using umbrellas and you will get an idea of the type you like. Just be sure they match, at least for the bridal party (1 umbrella per couple).
2. Designate someone to be your helper. You won't want all of your wedding pictures to have an umbrella, so it's a good idea to designate someone to come with you and hold the umbrella over you until the last possible second before a photo is taken. Their only job is to keep you dry.
3. Rent a tent. If you can afford it, rent one of those large wedding tents. It will be well worth it if you ceremony was planned outside. If you have to move your ceremony to the reception hall, your photographer will have VERY limited room and your images will be cluttered with guest tables and chairs. If you rent a tent, you can keep the ceremony separate from the reception and keep your pictures cleaner. This will also still allow for the large family portraits and the entire bridal party portraits. If you can decorate a back wall for the pictures, that's even better. The more you can do to compensate for the lack of outdoor beauty, the better. You might even be able to tear down a wall to allow for the outdoor beauty. This is an area where you can really get creative.