RaClips!
These little clips are a smooth and easy way to keep you bag secured to the rack while allowing simple and quick removal. 
Go here to purchase.

For a brief rough-and-ready demonstration of how the clips work, check out the video below:

-Mounted through the base of a handlebar bag, they are positioned to align with the rack platform's rails. The clips stay with the bag.
-A simple sliding mechanism releases the bag and can be accomplished by simply reaching under the bag to unclip. 
-No more fiddling around with velcro straps under the bag. Yes!
-Available for new bags and can be added to any existing bags.
-In conjunction with the Dock-IT™ your bag will be perfectly positioned and secure, while quickly and easily removable from the bike.
​-RaClips are designed to fit 5/16" rack tubing and will not work with larger size tubing
-Installation instructions download available here and here