Folk Acoustic Capo
What is the best capo for an acoustic guitar?
I have the YAMAHA Acoustic Folk Guitar
The Shubb brand capos are, without a doubt, the best capos out there.
The Planet Waves NS capo comes close, but the Shubb is still better.
Spring capos push harder on the strings with you put them up higher (since the neck gets thicker) which pulls the strings sharp in higher positions. The Shubb capos are adjustable to apply just the right amount of pressure, plus, they aren't big and bulky and stay out of the way.
Sure, you can't change your capo position in less than a second, but who cares if you can do that if it results in you being out of tune!?!
They make a few different kinds, with different curvatures to accommodate flat fingerboards (like on a classical guitar) to very rounded fingerboards (like on a Fender Strat). For a steel string acoustic you will probably need a slightly curved one.
They can be bought at most any guitar store for about $20.