I heard an author interviewed on NPR yesterday cite an example of another author who writes exactly 500 words a day. Even if he's in the middle of a sentence, in the middle of dialog. He stops. 

The next day it's easy to get started because he just picks up where he left off. Sure it might be trouble to remember all the detail, but it's a lot easier to do that than staring at a blank screen wondering how to get started. 

Why that's interesting -- that's more or less how I keep my momentum going, day to day, on programming projects. I try to leave off in the middle of something pretty easy, and leave good notes so I don't have to look for a warmup project for my next session.

PS: Sorry I don't know the names of either author. There isn't an easy way to search past shows on WNYC. It might have been yesterday, or it could have been the day before.