Go for a run, hit the gym or do some yoga. Runner's high does miracles for coming up with better approaches to tough problems. Even if it's midnight and you feel like passing out, just say fuck it and go for a jog - you'd be surprised how much energy you actually have.
This is good advice for starting a day, or ending a day. So many times when I get home from a long day I think I'm too tired to get any work done, but after a 30 min run or bike ride, my mind is ready to start hacking on a project again.
It's also not a bad idea after lunch. It's 4pm here and I just got done doing some pushups. Looking forward to doing some more before I leave.
