- As the old joke says, you hum it and I'll play it. Try humming, singing, or working things out on the kazoo before playing them on your instrument of choice. It's a good way of avoiding cliches.
- Use the chords. Guitarists in particular have a tendency to wander up and down a scale that sort of fits everything, with no consideration for chord changes. Ideally, if all other backing were to stop, the listener should still have a fair idea of what's going on harmonically.
- The small techniques matter as much as the flashy ones. I have a friend who can play classical piano at a good portion of the speed of light. Chopin, Rachmaninov, it's all the same to him. Unfortunately, his left hand is still very heavy on the bass notes, which rather ruins the effect. Sometimes, learning to inflect the notes exactly the way you want them is more important than being able to, for example, pull off perfect sweep picked arpeggios. Don't believe me? Ask Jeff Beck.
- Talking of whom... one for those musicians whose instruments habitually involve the use of plectrums. Try fingerpicking for more direct control over each note. You lose a couple of notes per second, but really, you gain far more.
- Learn some new music theory. A scale you haven't used before. A hideously extended chord. Anything might turn out to be just what you're looking for.
- Improve your ear. By working on recognising intervals, or less formally by working things out off CDs, you improve your chances of being able to play what you hear rather than relying on where your fingers take you.
- Pause between each phrase. Instead of a stream of notes, concentrate on playing distinct musical phrases. Silence can be just as effective as sound.
Above all, really concentrate on the sound of what you're playing. It's easy to get sucked into playing exercises, working faster and faster until the sound quality is just mush. Instead, make sure that the sound you're getting is musical and exactly the one you want.