I use the Trek Interchange system. I have the big daddy with the zip-out paniers. It's very stable even in rough/wet conditions. There are cheaper and easier ways to go, but this thing zips out all over the place. I have a friend who toured accross the U.S. with one. You have to be careful of the 20 pound weight limit, though. The panier straps require some kind of additional bungee or zip tie to reach down to the rear axle, though. Also, mounting a taillight to the back of the rack requires a very creative mind (or two) and some spare reflector parts and pieces.

http://cyclewerks.net/itemdetails.cfm?id=642

http://penncycle.com/itemdetails.cfm?ID=5364