Sometimes I think that the most important part of practicing is deciding what, specifically, I want to practice, and how, specifically, I should go about practicing it.
For example, take my current piece, as ripe a romantic cheese as you will ever hear in your life. My teacher picks it apart bit by bit and I try fixing each thing, but in the end he just declares "it doesn't sing." And he's right, it doesn't. So I need to work on it, but how? How do you practice making something sing?
I could play the piece over and over, kind of the million monkeys/million typewriters/million years approach. But something tells me that's neither efficient nor guaranteed to work. So what, then? What is singing? Is it intonation? Maybe, some, but my intonation is pretty ok on this piece already. How about dynamics? What is the purpose of dynamics? To help give the sense of a line. Ok, lines - what else really makes a line a line? In this piece, it's the use of the bow, and especially a bow that moves. Smoothly. At all times. Ok, then: smooth bow; the bow must never stop (except at rests) and it must be smooth through changes in speed and direction.
And there you go, there's the practice: play through the first few phrases, a bit under tempo, with the focus being "smooth, moving bow at all times." Out of tune, undynamic - don't worry about that now. You want that bow to be smooth, and moving, that's all that matters now. Do it 5 times, or 10 times. Something will happen, hopefully something good :-)