The Pnemo bags are awesome, but they're very expensive to use for storage of bulky items. But for keeping gear dry, they're the best things ever invented.

I've had really good luck with the space saver bags sold at walmart. I don't use a vacuum for it, just roll out the air and close. So far air hasn't leaked back in yet, just make sure the sealing surfaces of the valve and zipper are clean and you shouldn't have a problem with long term storage. The smaller size bags are a little fragile though, so you have to protect them to prevent little pinholes which will cause them to reinflate.