There's a problem that can develop for published writers when they finish one project and have yet to move on to the next. It's called "down time." You haven't put your foot solidly into the next manuscript and have the success of the last one to distract you--after all, there are book signings and appearances, the public clamoring for your attention! OK, maybe it's not that exciting. But it can be difficult to weather the lull that occurs and find that motivation to move past yet another blank page--or computer screen--and start to be creative again. You've probably also been editing for months without really having to create anything completely new. So what do you do?
The key is to stay creative in one form or another. When a manuscript is finished and you don't have that next solid project to jump into, use that time to develop other skills. Try a new form of writing: poetry, screen plays, satirical humor. Whatever. It can relieve some pressure in that you're not being expected to perform the same "tricks" over and over and it can be like a mental vacation from the usual workload. Or maybe just keep a journal for feelings and abstract ideas. Just write. It's like exercising and when a new inspiration hits, you won't have as difficult a time warming up those muscles again.