I recommend timed breaks (so you don't have to worry when your break should be over), walks, and relaxation hypnosis.
Ultimately you want to learn what kind of fatigue you're suffering from and figure out what kind of break is appropriate for it.
I recommend timed breaks (so you don't have to worry when your break should be over)
This is diametrically opposed to how I react to being timed.
So, it's well-known (or, at least, oft-thought) that you can't just work 16 hours a day; if you want to get stuff done, you need to rest from time to time. You have to take breaks.
Today, just now, I realized that I don't really know what the best way to rest is. If I want to rest, should I do something that's fun and interesting, like reading a fantasy novel? Should I do something that's boring, like building roads in Minecraft, so that work will seem comparatively interesting when I get back? (And besides, reading a fantasy novel is a bit of a challenge, at least for me, whereas building roads in Minecraft is trivial.) Physical activity probably helps, but how much? Do relaxing activities, like taking a warm shower, help more than just sitting there? How can I tell when it's time to take a break? How can I tell when it's time to get back to work?
(Suggestions for tags would be appreciated.)