1. If at first you don't succeed, take a step back and try again. Practicing can be a painful and frustrating experience for a student who is trying to do something that is too difficult. Instead of spending the entire practice getting more and more frustrated, break the task up into smaller or simpler sections until it is within the student's ability. For example, instead of trying to polish a song by repeating the whole thing over and over, spend time drilling the trouble spots first, and then play the whole thing through.
2. Be Consistent. It is far better to practice 15 minutes every day for a week than to practice for three hours the night before lesson. The practice that counts is consistent practice. I always encourage my students to practice 6 days a week, even if they are only able to practice for a few minutes on their busiest days.
3. If at all possible, practice at least as long as your lesson is. The length of your practice time will determine how much you have to show your teacher at your next lesson. Your teacher gives you assignments based on the amount of time you have at lesson. Students should never run out of things to do during practice time. Between new assignments and review songs, there should be plenty for students to work on! --If you do run out, I'm sure your teacher would love to give some extra assignments!