I do three things.
Stock up ideas before falling asleep. I'm one of those people that lay down and suddenly am more awake than when I was walking around. So I sit there and think of images to create. Last night I completed three watercolor paintings in my mind, but the problem is that I only remember one. Usually if I just focus on an idea and constantly repeat to myself that I should remember it, I will. Sometimes I'll sketch them out the next morning quickly, give a visual reminder of the full idea at the least, but I usually don't and lose the thought completely. I should start just sketching them all down and completing them when I have a block.
Walk around town. I don't do this much, now that it's -20 degrees out, but listening to Belle and Sebastian and walking around downtown for half an hour or so always rejuvenates some creative spirit somehow. I don't come back with any thought of what I want to make, but I do return with the need to create. Nearly doing anything physical will help me, but walking is just the more enjoyable choice. Sometimes going to work, shoveling snow, or driving around to do errands helps.
Listen to music that evokes a single specific feeling. A couple nights ago I listened to a lot of nostalgic stuff, followed by a lot of upbeat post-punk, which seemed to help me get in that creative mood. Other times I'll find that classic rock, ambient, turntablism, and on the rare occasion, even some electro industrial will help get my mind working again.