Talking through the problem out loud. Ideally with someone else who knows what you're on about, but anyone willing to listen will do.

If you can't find a listener just find a place where you can talk to yourself quietly.

Basically offer up a detailed commentary of the problem and what you have tried so far. The slow, methodical pacing of actually speaking out loud, as opposed to mile-a-minute thinking in my head seems to help my brain spot problems and come up with solutions.