Although many camera bags now come with cool plastic covers in case of rain, many still do not. If you know you are going to be outside in the rain, put a small hand towel inside your bag at the top, as to cover the equipment. Surprisingly enough this is enough to keep most of the water off of your gear! Remember this is not a water proof system, it is only to protect against a little bit of dampness.
**Towels do create some dust so keep close attention to that!**