Note that a pack cover will NOT keep your pack contents dry, either if it rains hard enough to run down your back and soak into the back of the pack or if you slip while fording a stream. The same is true of stuff sacks, which do not have a waterproof closure. You need either a pack liner or to use dry bags to keep your critical insulation dry.

Pack liners are available commercially or you can use 2-mil trash compactor bags (if you can find any that aren't perfumed) or (as Brian suggests) contractor trash bags. Normal trash/garbage bags (usually 1 to 1 1/4 mil) aren't strong enough to be reliable. Be sure to use a waterproof closure ("candy cane" closure, in which the mouth of the bag is twisted shut and then doubled back on itself). Check daily for possible holes and mend with duct tape. Some commercial pack liners are made like dry bags (waterproof material and dry bag closure).

I gave up using a pack liner after much frustration shoving small items down in my pack only to have them pop right back out thanks to the slick plastic. I switched to lightweight dry bags, one for my sleeping bag and one for my insulating clothing. They save me a good five minutes, as well as considerable frustration, packing up in the morning. The two Sea-to-Summit Ultra Sil dry bags I use weigh the same as a trash compactor bag. If I ever go back to a pack liner, it would probably be cuben fiber, which is far less slippery than either plastic or silnylon, as well as lighter (it really lightens your wallet, too!). BTW, be sure to test your dry bags in the tub occasionally to make sure they stay waterproof.

Jim's idea of seam-sealing your pack (may be difficult with packs with lots of seams and zippers) is good, but test the pack in the bathtub at least yearly before relying on it to keep the contents dry.

I use my pack as a pillow, so I like to keep the outside of the pack--at least the front part that contacts my sleeping bag--dry. I therefore use a pack cover as well. It's a 1-oz. cuben fiber one from ZPacks.com.