If a cup of coffee doesn't do it, and a short nap won't do it, it's usually because I've written myself into a corner, or haven't completely worked out a solution or thought the story through. What I've found works the best is looking at the work from a different angle. For instance, when I draw, I turn my sketch upside-down, and I can see my errors far clearer. The same holds true for the written word. But it's really hard to type with the monitor upside-down, and same with the keyboard, so I mix it up other ways. If I'm seated, I'll prop my keyboard on a box so that I'm standing at my desk. Sometimes I'll even march in place. This helps to clear the cobwebs. If that doesn't do it, I'll write longhand. For some reason, this method seems to free up a different corner of my mind and I am usually able to get past the problem.

I buy a lot of sound tracks albums from iTunes. Bram Stoker Dracula soundtrack when I need dark and dangerous, and when I'm writing the scenes where my protagonists are involved in an exciting espionage scene, theme music from Mission Impossible by Danny Elfman. (I also like Elfman's sound tracks from 9 and Wolfman.)

I don't recall who said it, but the best piece of advice ever:

"You can't fix a blank page." In other words, give yourself permission to write crap, then fix it. (This is especially valuable if you feel yourself blocked, or not writing, because your story is stuck.) And for those who work full-time or have kids or both, John Lescroart gave me this bit of sage advice, which I took to heart while working full-time as a cop and having twins: A page a day and you'll have a book done in a year.