We've all heard the saying "Practice makes perfect." Unfortunately, in a sport like tennis that revolves so heavily around muscle memory, this isn't always the case. If you want your lessons to pay off and your strokes to improve, you must use the technique you're learning whenever you play to reinforce the muscle memory. If after your lesson you ignore everything you've learned and use poor technique, you're actually reinforcing the bad habits and making it harder to relearn the correct muscle memory later. In summary, practice doesn't make perfect; perfect practice makes perfect.