It has probably rained at least some on every one of our trips. I think there was 1 trip where it wasn't even bad enough to buy ponchos, but it did rain nonetheless. It is funny that when we picture our perfect trip, no one pictures rain, but the reality is .... it rains! I've got tons of photos of old trips with us in ponchos and the memories are no less fond. I totally agree with the poster who pointed out that it could be worse. On one trip, it was windy and stormy on our first day. I tolerated it, knowing that sunny days were ahead. Well, it turned out they were BLAZING HOT .... in October! Looking back, that first day was the BEST weather of the trip.
You cannot control the weather, but as others have said, you can be prepared. I am actually a huge fan of the Disney ponchos. If it's threatening rain at the start of the day, grab a couple of ponchos from the store at the front of the park (where they rent strollers). They're pricey, but cute (as cute as a utilitarian poncho can be), and really effective - much more so than the econo variety. If a thunderstorm pops up, look for them at other shops. CMs pull them out and put them front and center when it rains.
Do a little research so you have ideas of some rain activities. For instance, when it rained in MK on our last trip, we headed straight for Mickey's Philharmagic. Concentrate on controlling the bits you can control and try to let go of what you can't.
Good luck!