I was just looking around and found your post. Over the years I have dealt with the same issue. I've had a number of different styles/brands of saddlebags. Currently using OSI soft Saddlebags. They have 2 wide velcro strap that go over the rear seat. I went to Autozone and found an "imitation" sheep skin/white fur pile for a single bucket seat cover. It's thick and very soft. I use it like a saddle blanket. Just put it on top of the back seat (fur side down) and put the saddlebags on top of the cover. Carefully position the seat cover and bags where only the soft fur is the only thing touching. The weight of the bags now on both sides, and the top straps, hold themselves in place. My tail pack then sits on top of the rear seat and side saddlebags and is strapped down. PLEASE! READ THIS AND LEARN FROM MY MISTAKE! YOU MUST MAKE SURE THE BAGS AND WHATEVER LUGGAGE YOU USE ARE VERY WELL STRAPPED DOWN. THE BAGS DO CATCH WIND AND YOU CANNOT RIDE THE BIKE AS IF THE BAGS ARE NOT THERE. About 10 years ago I was loaded down and ready to cross the country. I had my side saddlebags, a top tail pack on top of the seat the full width of both sadllebags. I thought I had a very good system of tying everything down. I got caught behind a slow moving logging truck. I down shifted to pass. I made it around the truck and as I was coming back into my lane around 130MPH, (Busa's get up there in the blink of an eye) The 2 front straps of my tailpack snapped, the 2 rear straps held. The G forces and wind blew the bag back and off. The rear straps allowed the bags to swing back, down, and into the rear tire. Upon contact, the bag went up and between the tire and rear fender, fully extending my rear suspention and locked my rear wheel. I was doing 130! I left a skid mark to be entered into the Guinness Book. I fishtailed left and right too many times to count. I went down on the right side and slid off the shoulder and into a small ditch. Fortunatly I was able to sit atop of the sliding bike only suffering some road rash from my initial contact with the pavement. I was able to keep it up until my speed came down to around 40 mph. My brand new rear tire wore down to the cord in that one spot. Don't take this subject litely. Think about it and be certain. Better safe than sorry. (damn, I sound like my Dad. Sorry).
Hope this helps.
BUSA1 (Mike)