Back when I used to teach guitar, one thing that came up a lot during lessons was that guitar players wanted to "get stronger." And in their frustration to get stronger, they would try all sorts of things--like playing with rubber bands around their fingers, all sorts of stuff. When you watch a beginner play, they press 10 times harder than someone who's been playing for 10 years; they waste a lot of their strength and hinder themselves and their ability to play by using too much force. But when you're hammering-on and pulling-off, it's actually all about accuracy, not strength. Find out where the perfect spot is to hammer-on and to pull-off, and then expend energy only to do that, and get rid of everything else. Ask yourself: "When I put my finger down on a string, do I feel that it's the most efficient spot? Should the string be a little bit more in the center, or off to the side of my fingertip? Am I feeling the finger bone below that?" And that needs to be addressed on a personal basis because my flesh, my bone, and my calluses change from day to day. Yet they're unique--everybody's hand is different; everyone's fingers are different. That approach suddenly relaxed my muscles and tendons and freed me up to be more musically responsive. In other words, you could really start whipping your fingers around in real musical rhythm, instead of thinking about brute force.

I'd have my students do a simple sort of nonmusical exercise (Fig. 1), being very careful to hammer-on absolutely perfectly--getting the best possible finger placement--then picking as little as possible on the initial attack, trying to eliminate the sound of the pick entirely. Don't worry about strength. Concentrate on sound and placement. If you feel any pain, stop. Make sure you're always totally relaxed, and that there's no pain, tension, or stiffness whatsoever.

Fig. 1a
 

Another version of that would be to hammer-on/pull-off between the first two fingers, and then hammer-on/pull-off between the first and fourth finger, continuing across the strings (Fig. 2). You can also align your fingers one per fret--first finger first fret, second finger second fret, and so on--and do trills. I used to do that a lot, fluttering between first and second, first and third, first and fourth (Fig. 3A), then doing first and second, second and third, third and fourth (Fig. 3B). You can spend all day coming up with alternate versions of this exercise. Don't worry about the notes. This will allow you to turn off certain anxieties like, "What key am I in?" Just blindly come up with every finger shape you can and vary the amount of stretching.

Fig. 2

Fig. 3A

Fig. 3B

When I taught these methods, I had to make sure that they didn't take up too much of a student's time, so I would also go through two- and three-octave major scales. I would have them do the scales on one string, and then I would have them play the scales without any set fingering--as high and as low as you can go on the guitar. And the easiest way to introduce the idea seemed to be doing a three-octave-plus scale, four-notes-per-string (Fig. 4). But I was always afraid of doing that because practicing that way sometimes reinforces the exercise pattern so much that it creeps into their actual music.
 

Fig. 4

My experience taking some lessons with Lennie Tristano, the great bebop piano player, really made an impression on me. He couldn't stand it when he heard people playing anything that sounded like it came from an exercise book. He thought that was the most ridiculous thing ever; it was just horrible to him. So his style of teaching was to not have a set fingering or a set picking pattern, but to play everything everywhere on your instrument--as high and as low as you can go, in every possible place, harmonized in every possible way, with absolutely no mistakes [laughs]. He'd say, "If you wanna pause for 15 seconds before you play the next note, that's okay with me. But don't play a wrong note. Wrong notes don't work." If you made a mistake, the lesson was over, and that was it. And I remember having a couple of six-note lessons, I swear.