I'm loving those Hefty 2.5 gallon bags. They beat out the expensive SpaceBags for keeping air and water out. One bag lasts for months! Ya just stuff your clothes in, press out the air and presto. A very flat pack!
I did that last year on Cycle North Carolina -- at least when I packed everything up at home before the trip. Once I got into the ride, I tended to just throw everything in my main bag and not worry about it. But those big zipper bags sure worked well as a poor man's "space bag."