Using a metronome, playing just one note on any string, or muted, start with whole notes, then continue to half notes, 8th, 8th triplets, 16th, 16th triplet etc...continue this pattern as far as you can. gradually speed up the metronome. Also, as mentioned above, practice scales focusing on acuracy. then, pick each note 2 times, 3 times, 4, etc, digital progression is also good, and commonly used it classical music.