Practice.

The best practice occurs when the body is warm and running like a car on the open road. Too many people get disheartened in the first ten minutes or so of their practice and give up.
Practice should start with at least ten minutes of warming up.
Scales and arpeggios are one good way to warm up. Try to play throughout the range of your instrument. Use a metronome and use harmonics, and open strings, to check your intonation.
Most improvement in your playing will not come on one day, so use repetition to slowly overcome obstacles; plan for the day when these impossible tasks will become easy. Also take time to cement your past gains and to enjoy where your playing is at.
Remember that your tone, how you sound, is everything.