When I'm lacking motivation, there are several things I generally do. I may watch DY DVDS or other videos on YouTube. Sometimes, I'll watch old WSM DVDs. Then there are times that I'll just sit back, relax, and picture in my mind some of the times that I've noticed progress. Then I'll think about how that made me feel, hold onto to that feeling, and go hit the weights...also, there are the random days that I tell myself to get off my lazy @ss and get with the program.
Try to discover what works best for you and roll with it....personally, I think MuscleInk suggested a great strategy.