I have a 30 degree down bag from Kelty, so it's not too big to begin with. But I've had good success with the Sea to Summit Evac dry sack. It's not a compression sack, but it packs down pretty well--you roll it from the top and buckle it closed. I've only used it for two seasons, so I can't really tell if it's had an effect on the bag, but I doubt it. The thing I like about the evac sack is that it's oval-shaped, not round. Won't roll away! Not that I've ever inadvertently set a sleeping bag down on a slope and had it roll into the lake... :o

Oh, and while it is waterproof, I do line with a plastic bag, just to be safe.