For me it is anything that helps me not fix on the problem. Standing in front of the computer, staring at the code always pushes me in the same direction; once I am up to something else, but still 'lightly' thinking about the problem I come up with better solutions. It's almost as if I am thinking about it I can't think of anything valuable; once I stop thinking about the issue at hand everything just happens to fall in place.

PG has theories on why: http://www.paulgraham.com/top.html

For solving problems showers are faster, but walking is better for higher level problems requiring greater thought. A shower is great for knocking out a block quickly, while if I have a really hard problem a long walk is better.

I wonder if there's a patent on a treadmill shower desk.

Now just add a fridge and a microwave and you'll almost never have to get off.

And sleeping on a problem.