Burnout is real and one of the most dangerous things you can do is train yourself to feel guilty about not working. So you need to remember to take breaks. The break in a Pomodoro is a good one, but I also recommend taking a larger break (like 30 minutes) after completing three or four Pomodoros.

I have found it useful to have lists of activities I can do during short and long breaks (links go to the corresponding Workflowys).

Scott Young's The Little Book of Productivity

Here's my summary of Young's book. And here's a list of the productivity resources I've found most useful.