Though there is no doubt that a lot of practice is a good thing, how one practices is the more important skill of a musician. Here are some suggestions as to things to do while practicing:
1. DO use your pencil. The pencil is the most powerful musical instrument ever: feel free to use it whenever necessary in personal practice, a music lesson and even during orchestra rehearsals. Make your markings clear but concise – often symbols are better than words.
This following example is good. LH is the standard abbreviation for the Lower Half of the bow, a short line is used to show that the finger moves down, and only the important changes in position are marked. Some people might argue that having two possible fingerings is a bad idea, but I feel if it is clear, and you are clear, it's fine.
2. DON’T try to do everything at once. Pick out something to fix (such as bowing, intonation, speed, strength of the fingers), get exercises to help that skill, and focus. Then the next practice session try to maintain that skill, and find something else to fix.
3. DO ask questions. Having good questions and the search for good answers is a positive way forward. For example: Is there another way to do this? Am I breathing with the piece? Why do I feel stiff in my arm?
4. DON’T just play through the piece over and over again.* Without fixing the mistakes, all this does is to make sure one’s fingers keep remembering the mistakes. I suggest that in regular practice, just play through a piece two times – once at the beginning of practice to find out where the problem areas are, and at the end of practice to see your progress. Closer to a concert, one can play through a few extra times.
5. DO keep track of your practice. In two ways: the amount of time you have put in, and also the number of things covered. Put it some place visible, like on a notice board, or Post-Its on your wall, to see your progress. Have a goal in mind, as in to play a piece well by a certain date.
*A common misunderstanding is that the Suzuki Method teaches this, to play a piece over and over. Though I don’t use the Method myself, it should be pointed out that Dr Suzuki has said and written that it is only something correct that should be repeated many times.