Arkel makes a great rack that clips on to the seat. Literally takes seconds to attach. They have a couple of bags that attach very securely to that rack.

For clothes and minor stuff, the saddlebag racks like Relevate or better yet, Bolderbikepacking are excellent.

http://bolderbikepacking.files.wordp...rd-touring.jpg

I just bought my second bag from Greg at Bolder Bike Packing and they are fantastic, very light and you can put an amazing amount of stuff in them.

Bolder Bikepacking Gear « Rugged lightweight bikepacking gear for racers and adventurers.

For a road bike depending on how aggressive it's geometry is, you will want to work towards minimizing the weight on the rack/seat post because of how it will impact the handling of the bike. You'll have to experiment and see.

J.