From Terry’s book, Shred Is Not Dead:
Technical guitar playing is probably 99.9% in the mind. It’s not really what you practice, it’s how. You must develop your ability to concentrate to an immense degree. This is easier said than done. Try this exercise within an exercise:
Take one short lick or sequence… Loop it – play the example over and over again. Don’t think about anything but what you’re playing. Feel the pick hitting the strings. Is it dead on? Are you getting a full sound on every note? Feel the tips of your fretting hand. Are you using just enough of the finger to get a good sound without being inefficient? Are your fingers springing off as quickly as possible without cutting the sound off? Are you lifting them too high? Try to sense all of this at once. Keep going.
During the looping exercise, are you thinking about what happened in school or work today? Are you wondering what’s for dinner? Are you thinking about what’s on TV? Are thou thinking about an object that’s iin the room you’re in or an event that’s happened to you? Most likely, the answer to one or more of those questions is ‘yes’. Or maybe you were distracted by a sound, like birds or cars outside your window. The answer to all of these needs to be ‘no’.
You absolutely must concentrate completely on whatever you’re practicing at a particular time. Essentially you must ‘be’ the exercise. The mind wants to wander – and certainly will – unless you take control.