Anyone who's spent enough time practicing anything - piano, basketball, karate, painting - will tell you that there are tips and tricks of practicing that apply all across the board. If you look at a guitar lesson the right way, it becomes a piano lesson. (Right now I'm developing a whole set of guitar lessons based on practicing jump shots. Stay tuned for further developments.) Here's a piano lesson from Jeff Gardner's extremely complete Jazz Piano: Creative Concepts and Techniques:
"Make up your own exercises and etudes. These should address the technical problems or pieces or musical concepts on which you are currently working. It is a tremendous reward to write something, look at it and say, "how am I ever going to play that?," and then practice it until you can play it. This is one of the best ways to arrive at your own style of playing and writing music."
There are at least two great lessons here in addition to Jeff's intended lesson, and they can be piano lessons, guitar lessons or bagpipe lessons. The first one: "Make up your own..." Music is the only art form I can think of where people are not regularly encouraged to create rather than imitate. Unfortunately we think of composition as a separate idea from performance (not necessarily true), so when we say "piano lessons" we really mean "performance lessons." And teachers, who may have had actual composition lessons and may have great respect for the skill that composition requires, may not feel up to the job of teaching you and me how to write. I say, do it anyway. You'll learn the obvious lessons Jeff intends you to learn. You'll engage an additional piece of your brain, which is always a good thing. You'll enjoy it. And - who knows - you may write good music.
The second one: "say, 'how am I ever going to play that?,' and then practice it until you can." Set yourself an impossible goal and then prove that's possible after all by achieving it. My favorite personal flub in this department is to do (a) or (b), but not (a) and (b) - I set an impossible goal and then fail at it, or I set an entirely too possible goal, and then achieve it, thereby accomplishing nothing. But what a great feeling it is to accomplish something you thought was impossible. That's as important a lesson as learning half-diminished arpeggios or super locrian scales in groups of three.