Recommendations for a duffle bag?

[Teecegirl]

I recently strapped my gym bag (duffle bag shaped bag) to the back of my bike.  I used those elastic tie cords to hold it in place.  Well, by the time I got home, it had started to slide to the side.

I'm wondering if anyone has found a good duffle bag to use or a good way to strap one to the back of their bike.  I'd love to commute on the bike, so it's important that I get something that's about the size of a gym bag and won't slide around.

[indestructibleman]

a bungee net will hold things in place much better than a cord.

http://www.motorcycle-superstore.com/item.aspx?style=4524&department=645&division=6