I think most people will say "don't do it", and I tend to agree since it does hurt quality, but a few are: microwave for a few 5-10sec cycles, put on a piece of tin and place 6-8" above a light bulb, put in a mug on a cup warmer, oven on a cookie sheet (yuk!)...
I've tried a few in a pinch, never really liked the result though...YMMV