Unless you're a totally fair weather rider, you will eventually get caught in the rain. Not only will you get wet if you're not in rain wear, but so will your gear. There's a simple solution to keeping your stuff dry, and that's to use a dry bag (sometimes called a wet bag). These bags are not really designed for the motorcycling community, but instead for the those who prefer white water rafting, canoeing, or kayaking. That doesn't mean, however, that they won't work well for us. It does mean you're not likely to find them at your local dealership, but you will find them about anywhere that sells canoes or kayaks.
Dry bags come in several sizes. The smallest are designed for cell phones, and the largest to hold as much gear as you're likely to want strapped to your bike. For motorcycling I find a 5 liter bag to be about the right size, but I also own one that's about half that size. The 5L is roughly as big around as a 3 pound coffee can, and a bit taller once you have the top folded down. If you're planning a long trip, you may want to consider the larger sizes. The brand I favor is called SealLine, but there are other manufacturers. Expect to pay around $15 for the 5L, and more for the larger bags.