I'm having some balance issues with my heavy backpack falling off the cheap "clamp-onto-the seatpost" type rack that I used to have. I tried more bungy cords and that didn't work. The backpack kept falling off during my bumpy, twisty, urban commute ride.

Anybody know how to deal with this? Is there a extremly strong rack or tie-down method that I can get that would allow me to basically tie the backpack to the rack, get to my destination, and quickly get the backpack off the bike? Maybe, I should get something other than my old setpost quickrelease rack.