[michaelflux]

Spend a bit of money on some desk plants. My desk looks like a little rainforest. All the extra green keeps you happy, you get a bit of extra oxygen, and gives you something to take care of to distract you from staring at nothing but your screen.

Beyond that I'm shocked no-one has mentioned getting either the comfiest chair you can find, or a standing desk. Either of those two will make a night and day difference.