+1 for Rokstraps. Think I've got at least two of each size.
http://www.traveldriplus.com/shop/misc-luggage.html(Bought all mine from this bloke - keep an eye out for him at shows. Great kit, sensible prices and knows a bit about bike touring. No commercial attachment - just a happy customer)
Anything heavy gets two rokstraps in an "X" configuration, followed by a cargo net over the top. My theory is the Rokstraps hold the pack to the bike, and the cargo net stops the pack moving out from under the straps.
Cargo net also gives you somewhere to stash your waterproofs on that odd occasion the sun comes out.
I tend to go a bit overboard with bungies, Rokstraps and cargo nets - it means you've got extra should you make any purchases along the way.
If you are using waterproof bags (e.g.
http://www.traveldriplus.com/shop/motorcycle-luggage/motorcycle-dry-bag.html), it's important you get as much air out as possible.
First time I used a large Ortlieb bag on the rear seat, I had two Rokstraps holding it on. As the trapped air slowly escaped (I'd packed the bike the night before travelling) the straps ended up loose. I nearly ended up losing the pack a few miles down the road.
Nowadays, instead of using the roll-closure, I just fold the opening under the pack and attach it like that. Air can escape when you tighted the straps, but I've yet to have any rainwater get it.
Some waterproof bags come with an air valve in the bottom, so it can't just be me that's experienced this.
