Go to the motivation page under articles. There are a lot of articles that have helped me when I'm having a bad day. Reading Success Stories also really helps. Have you tried blogging about your motivation issues or what you think causes them? When I do that my SparkFriends and team members always leave encouraging notes.
When all else fails, I tell myself, "This is what I need to do. I know I need to do it and I have time. Just do it."