I'm a big fan of the Topeak series of bags. They have a bracket that attaches to the seat rails allowing the bag to be easily detached. They come in various sizes from small (one 700cx23 tube and a CO2 cartridge and tire levers all the way to huge. They all fit the same bracket so they are easy to move from bike to bike.
__________________
Cheers, Mike
-Stupid hurts....ride safe