Go home. Go to bed. Read a book about something technical but different. Take a shower. And maybe set a deadline (works assuming the brain is already familiar with the problem and can have some relaxed time in between learning about the problem and having to solve it). Chit-chat on Perl Monks. And 75% of the time, if I start writing the problem as an email to a mailing list or a post to a site like PM or to a newsgroup, by the time I'm done I've solved the problem on my own, because I had to explain so much of it anyway or I've double-checked part of a manual I hadn't thought to look at before.