Sometimes I just let it go for awhile, take a break.
Other times, I pick a UFO and say "Today I will do SOMETHING on this UFO, no matter how small." Maybe I plan to sew at least one seam, or square up at least one block. I hardly ever stop at that one small thing, though, and pretty soon the end is in sight, and I get anxious to finish. Having a finished project to admire usually gets me thinking of new stuff I want to do.