I'm seconding the greenfield stabilizer. They're very nice; I had one on my Trek 520 before I crashed it. Rebuilding my bike on a new frame, I didn't reinstall it (as it doesn't fit easily on my Long Haul Trucker Frame) and don't miss it a bit, but the stabilizer worked very well and didn't harm the finish with reasonably careful installation. They are very convenient for loading stuff into panniers. The stabilizer is very easy to kick open.

The regular Greenfields also are fine if they fit in your frame well, though I think the stabilizer is a little more...stable. You can use old inner tube and/or electrical tape to protect your bike's finish. My local bike coop has a seemingly infinite supply of these for next to nothing.