A bevy of illustrators and creative artists tell what they do when they hit their equivalent of Writer's Block. Some good ideas in there.
I don't have that problem often but when I do, I spend some time in utterly self-indulgent writing, writing (or just thinking through) a story not for sale or publication that has special meaning to me…and maybe only to me. That seems to unjam me enough to the point where I can then write whatever I can write. Another method is to just look at my American Express bill.