RainmanP

VR,
I have always found the same thing with seat/saddle bags; it is hard to really cinch them down so they don't sway. The best solution I have found, which works quite well, it to use the second method I describe above. Once you snap the buckle, you can pull the loose end and cinch that sucker down where it won't move. To get it good and tight you have to hold the strap to keep the whole thing from just sliding, snug it down, slide the strap over to get some more space, then hold it and cinch it again. Easy to do, hard to describe. I then tuck that loose end out of the way. Not only does it keep the bag from swaying and bouncing, it takes stress off the regular straps. Do you have a handlebar bag? That's what I use on my second commuter, and I like it better than a seat bag. I don't actually even use the mounting straps on the bag. I have wrapped the bag neatly in waterproof black cordura, taking care that none of the folds provide an entry point for water, then use two of the straps with snap buckles to cinch it up tight to the bar. It does not move.
FWIW,
Raymond