Do what many hikers/backpackers do. Line your pack with a heavy plastic bag. Put your stuff in and roll the bag top down to create a water proof seal. Most use a "contractors bag. It's simple, cheap and it works.
+1
I'm currently using the plastic shopping bags for my clothing inside the backpack trick. A little more ghetto than the contractor's bag but still fairly effective. I also take a second plastic bag and tie it around my seat. Put the knot in the front of the seatpost so it doesn't obscure the rear tail light.
