In the Sierra Nevada, usually a poncho that covers the pack is enough, at least in the summer. I've just moved over to using the Gatewood Cape from Six Moon Designs. Since I use a tarptent for shelter, it's also insurance in case I need to cover my down bag from rain splashes (and can be set up as an extra shelter or vestibule if needed). For me it's a piece of mind vs weight thing.
When rain is more likely, I often combine this with either a trash compactor bag as a liner; my food bag or bear canister sits on top of it to hold it closed. Inside that, I have my down and clothes in silnylon stuff sacks.
I've also used a WXTex Pneumo-something (their lightest version, reviewed well on BPL, not the heavier ones a la Arctic1000) 15L for my sleeping bag, jacket (both down), and spare clothes...it's heavy compared to a stuff sack at 5 ounces, but the purge valve lets it compress very well without straps, and again, piece of mind weighs nothing. (I did have to patch the Pneumo bag after a very gentle rub against a stucco wall in Thailand - they're pretty fragile - but two small pieces of duct tape and silicone sealant did the trick. Inside a pack, no worries.)