#1 Alway practice when you are fully awake and ready to learn. Know the times you work best: Mornings, Evenings or Afternoons.
#2 Make a routine and follow it! Set aside time every day to practice at least a half an hour. Your best practice time will not be How Long, but How Well.
#3 Make certain that you warm up before you play. You are using muscles so take care of them. See my "Warm Ups" page for ideas!
#4 Plan your practice time. In a notebook, set your goals and think about what you need to do to get there. Do you want to perform a particular piece of music? What do you need to know musically and technically to make this happen? Write it down and keep the notebook on your music stand to reference and keep you on target! Also, write down encouraging comments and even the goals you have achieved. If you are just starting lessons on the violin, write that down. Starting something is the first step and it is by far the most important!