When I am backpacking on a weekend trip and I know there is going to be a chance of rain, I bring one of these rain covers along. Outdoor stores and backpack manufacturers make these, and I have found most to do the job quite well. I use a Gregory brand rain cover for my Gregory Deva 60 backpack, because the fit is very good.
Specialized backpack rain covers are heavier than plastic garbage bags, but they do the job better. They have a closer fit to the pack, and are of a sturdier nylon construction so they don’t tear if a branch from a tree snags your backpack.
REI has a generic pack cover called the Ducks Back Rain Cover, and it fits a variety of packs. 
Luckily I have rarely used my rain cover, but on the occasional downpours I encounter on backpacking trips they come in very handy.
Happy hiking in the rain!
Nicely done, we can all use tips on how to stay dry, or as dry as possible while enjoying our hikes in the rain.
HL does such a good deed in helping us to fully enjoy the outdoors in so many ways; your reviews are Terrific, especially with the pics and suggestions that we can use.