Tom, I've just got one of the new Planet Waves capos (the one with a little knurled screw that you gently tighten when the capo's on. Works a treat, you can get EXACTLY the right pressure, doesn't noticeably bugger the tuning up, and it's slim and light so it fits snug up behind the fret and I can still get my bunch o'bananas in to play OK. For fourteen quid it's well worth a try!
Cheers,
John