I get off the keyboard and pick up a pen. I wrote my way out of a few plot problems in The Man I Love that way. Something about the mind-hand connections gets the flow going for me. Also getting away from the desk where the work and the problem is staring me in the face. I take my notebook and a pen, go sit on the couch or in bed or in the garden and I just write stream of consciousness.
I have a secondary trick of thinking really hard about the problem when I'm falling asleep. And I sort of offer it up to the Great Muse and ask "Little help here?" This doesn't always work but once or twice I've dreamed a solution, or figured it out as I was pouring coffee the next morning.