How do I keep the contents of my pack from getting wet?
Throw on a pack cover. A simple waterproof cover keeps the inside and outside of your pack dry, and minimizes the added weight of a wet pack. It’s also a good idea to put your sleeping bag and clothes into a heavy duty, plastic garbage bag. Zipper storage bags also keep smaller items dry and organized. Finally, store your pack in the tent vestibule, or cover it with an extra-large garbage bag or pack cover at night.

This is one of 6 questions answered in a good article from Backpacker.com.
Read the entire article here