Some ideas in no particular order:
1. Take a break for a couple of days.
2. Buy a new CD.
3. Buy a new instructional book or video.
4. Buy a new piece of gear.
5. Tune up your guitar (clean, strings, neck, action, intonation, etc.)
6. Transcribe a favorite lick or solo.
7. Force yourself to learn a DIFFICULT exercise.
8. Work on a technique that you never really work on.
9. Justin890's suggestion.
10. Listen to jazz.
11. Record.
12. Jam with others or join a band.
13. See live music.
14. Listen to James Murphy. <-- This always works
