My simplest and most often utilized method is simply to not paint. : ) If I let myself get bored for long enough (and the length depends on how blocked I am, and how long it's been since my last break), ideas just start popping into my head and I can't wait to get in my studio again.
If it's a little block I might just go for a walk. That usually clears my head sufficiently.
If it's a big block, I'll often spend some of that non-painting time with:
cleaning my studio
piterest and art magazines