I think we all get the attacks of "no motivation" from time to time.
I do like most of the aformentioned posters- vary my route. I also take the time to explore my city and find new places to go- there's a lot to explore here in Chicago.
I also set goals for myself- either trying to improve on my time, or improve on my speed, or improve on my distance. I also have a goal at the end of the summer to do my own century- I keep my eye on that prize at the end of the summer, and the fear of not being prepared also keeps me motivated to make the most of my rides.
There are also days when I don't ride at all. Sometimes, I feel a bit burned out from all the riding, and I need a break. I don't do any other sports, just walk from point A to point B. Then after a few days, I get on my bike, and as soon as I swing my bike over the top tube and take off, I feel like I must have been crazy to not ride- the ride is always a good one after the break, because I'm so happy to get back on and ride!