Great post at Lifehacker about the benefits of getting up and away from the desk for a quick walk. Not only do I get my 20 or so minutes in each day (to and from work), but I make it a point to go outside at lunch and walk around the park a few times as well. Clear your head in a big way. Energize yourself.
I'll say the same thing here I said there : What you're trying to do is make yourself "zone out". Because when you do this, you empty your mind of all the stuff that's gotten in the way. When that happens, the good ideas tend to pop in like lightbulbs.
How you do it is entirely a personal thing. I like to start with an audiobook or other spoken word, and then let my mind wander. When I realize that I don't know what's going on in the book because my brain is off doing other things, I switch to music and then pursue the thoughts more fully. (This, by the way, is also a trick I use to get to sleep at night.) Others, though, might prefer silence.
What works for you?