Some good habits:
Find your good time - two to two-and-a-half hours like helm said is the ideal time I've found as well, and you must set out with the goal to accomplish something. Some of my strongest pieces are literally 2 hours studies.
DON'T overwork a piece - if you have gotten to a point where the piece looks good and you are no longer learning from it, stop! Move on to something else.
DO review your work - You will learn most form your mistakes.
DON'T give up - learning is about discovering new solutions; if you never try for those, you'll never find them.
DO vary your subjects - Drawing the exact same thing every day and nothing else will only make your hand hurt and your art stop getting better.
Push yourself - intentionally pick things that are attainable, but new. If you've done it before, you won't learn much. This can be as simple as changing the colors, or the lighting, or the angle, but you'll want to explore with each piece and pick things you aren't very good at in order to learn.
Enjoy your art - if you constantly go around saying that you'll never be as good as what'shisface, you'll end up right. Be proud of each accomplishment while studying the good things and bad things that the master's do.
'sabout all for now i think.