You hear a lot about writer's block and how it can stop a writer in his or her tracks. Me, I've never really been blocked. I've had problems getting the energy and enthusiasm to finish a book sometimes, but more often than not that's because the story just isn't working and subconsciously I know it. When that happens, the best thing to do, I find, is to start writing something else, even if it's just for a day or two.
What I find helpful is to have ideas on the back burner, stories that I am constantly thinking about but don't do any real work on. But when I find myself losing enthusiasm for the project in hand (which admittedly doesn't happen very often) then I have something to fall back on.
And a great way of keeping those fallback ideas in play is to get a cover and title lined up, ready to go. At any point in time I have a dozen or so titles and covers that will provide me with inspiration. Here's a few I have lined up for my Jack Nightingale supernatural detective series. Most of them cost very little, but I'm pretty sure any half-decent writer would be inspired by any or even all of them. Certainly if I got bogged down in my current project - a stand alone terrorism thriller set in London - then it's the easiest thing in the world to sit down and write a story to go with one of the following covers.