When I was a teenager, the "tight roll" was in style. Nowadays I use it nearly every day to keep my pants out of the chain. It works better than anything else I've ever tried, is totally free, and you don't have to carry anything around with you.

http://en.wikipedia.org/wiki/Tight_rolled_pants

And here's a video about how to do it: http://www.youtube.com/watch?v=tJLpyL3LUbw

It's the best video I can find but the guy's a little sloppy about it, but you can get the idea. It's kind of like tucking the sheets on your be for nice, crisp corners. If you're careful and methodical, you can get a good, crisp, clean roll that stays nice and tight all day.