Dress appropriately! Dress warm and waterproof as much as possible, especially shoes. If a poncho is as good as it gets (and DLR ponchos are pretty good -- and can be replaced if they tear, just ask at the shop registers), then bring extra socks. Take lots of photos -- the parks are lovely in the rain, especially at night when the lights reflect on the water. Some rides won't run in the rain, e.g RSR, tea cups, Luigi's, etc., but others will be walk ons (depending on how hard and how long it rains). Take advantage of this! Enjoy hot chocolate while watching the rain -- so fun!