I am surprised that so many recommend the spring loaded capos. My advise would be to avoid any spring loaded capo. They press the strings right down to the fingerboard and pull them sharp. You need something that is adjustable. Tighten it JUST enough to stop the buzzing and apply it as close to the bridge side fret as possible. I always tuned my 12-string down 2 frets and left the capo permanently on the 2nd fret.
I've never tried Rick Fielding's advice about notching the capo, but I've never gotten bad advice from him; in fact Rick was the one who told me about Mudcat Cafe.
Pete Seeger uses a Shubb on his 12-string.
Another nice thing about the Shubb is that you can't clamp it on the headstock of your guitar. I hate the looks of capos or tuners clamped to the headstock. The only thing worse is a capo AND a tuner clamped up there, especially on a banjo where the headstock is often the prettiest part of the instrument.