It doesn't really help to pick a practice tempo and stick to it. What helps is to pick a practice tempo and do a bunch of repetitions, and then pick another tempo, and do a bunch more repetitions. Different tempos, either on different days or on the same day, depending on what you have time for. Ideally, for a passage that you have been working on a while, warm up slowly, then practice a series of repetitions at your fastest comfortable speed. Then put the metronome on, slow it down a little (maybe one click), and do another series of repetitions, then down another click and do another series of repetitions. Please notice that you slow down as you continue to add repetitions, not speed up. I don't know why this works, but it really smoothes things out, as well as giving you all the speed and accuracy you need.
(Via Lifehacker - read the comments too)