Programmer Matt Moore says some days he cranks out work, and others he can barely get anything done—and shares some tricks he uses to make today one of the productive ones. My favourite is his suggestion to leave work with something small broken so you have an easy place to start the next day. Moore writes:
Programming effectively requires me to have a lot in my brain at one time—which I "load" when I start, and which rapidly dissipates when I get distracted or stop. That means that getting started and "into the zone" is the hardest part. What makes it easier to get started is if I have a simple task to complete that gets me in the zone. So, any time I stop (lunch, or in the evening), I intentionally break something so I can get right back and fix it—when I get back to work, I'm not only anxious to fix it, but I'm in the zone after I'm done fixing it.
Seems like this would work whether or not you're writing code. If it's not leaving purposefully broken code, it may be just writing yourself a note about the first thing you need to work on.