Here's a technique I've been using a bit: take the least well developed chapter of your book and work on it until it is the third least well developed chapter. The way I do this is to take a sheet of paper and write out everything I know about what is going to be in the chapter, then sit down at the computer and write.
Now take the new least developed chapter, the one that has been left in the dust, and do the same thing again with that one, and with a third chapter.
This also works with completing chapters. Take the chapter that is closest to being done and bring it to completion.
Since beginning and ending are separate skills, it is refreshing to switch between chapters sometimes. The other main skill is just "going on," working in the middle of a chapter that is neither incipient nor near completion. That is the hardest stage, the one least abundant in satisfaction. What I recommend here is to wage a stealth attack, taking the chapter quickly from its initial stages to its final ones.