Watch the Hammer-Ons: 1 online guitar lesson by Muriel Anderson from 1-2-3 Fingerstyle Guitar
A section from "Nine Pound Hammer" makes a nice hammer-on exercise. It's an E chord with the pinkie hammering-on to the 2nd fret, going to an E6 chord.
Keep the thumb going in tempo while playing the hammer-on. That's the challenge!