Writing is much like being a single founder. I find the single most important thing is to be aware of how motivated you are, and to adjust the work accordingly. If you try to work on hard stuff when you're low, you'll just procrastinate. But if you start working on easy stuff, getting something done will put you into a better mood, and you'll be able to move on to harder stuff.

Hemingway used to leave something easy unfinished in the evenings, so he had something to start with in the mornings. That works for me too, with both writing and hacking.