I'll second Will's suggestion of practicing with a metronome. I often play in jam sessions with folks that I can tell practice/play alone without a metronome. They tend to speed up on the parts they know well and slow down on the more difficult parts or parts they don't know as well.

I'll also recommend an aphorism from my friend and a great old-time banjo player, Dave Landreth, "Practice slow to play fast." When learning a new tune or a new lick, practice it more slowly than you think you need to; once you've got it nailed you can bring it up to speed. Practicing something new at speed leads to sloppy playing.