Maybe it's the therapist in me, but I am a strong believer in goals. Whenever I feel my self losing steam at whatever stage in the writing process, I jot down a few goals. Where do I want to be with this project in a month? In three months? What do I want to accomplish before the year is out?
I find that looking at these goals helps me to find my stride again. It's a reminder that I have places to go, and I'll never get there by sitting on my butt. It also has the added benefit of practice meeting deadlines. One day, gods willing, I will have an agent or publisher to appease. I'll be damned if I want to fall short in that area, after working so hard to get there.
I've found it beneficial to have a plan. During the query process for one novel, I kept plugging away at another. There is merit in that old adage to just keep writing. I may have been discouraged after the slew of rejections on novel A, but I was just finishing up novel B and ready to start to query that project. It has helped me to keep my momentum. As I begin to query novel B, I have already started on novel C, and have some very solid ideas for novels D and E. Take a number guys, I'll be right with you.
I'm also learning to multi-task. I plan out stages, rather than whole works. For example, work on rough draft of novel B. Then while that one is out with beta readers, step away from it and work on plotting and character sketches for novel C. Go back and edit novel B. Start writing novel C while B is in query process. I have always been afraid to switch projects- afraid it will keep me from finishing my story. However, I'm finding it is helping to keep the ideas flowing, as each little break from a project lets me return to it with fresh enthusiasm and ideas.
Are you a planner? Do you set goals? Or do you just work at your own pace?