I know exactly what you mean. Some days I can work all day and still feel like doing more work, and other days I can barely bring myself to do anything.
The key for me is to just get as much done as you possibly can when you are motivated. Luckily I'm a college student, so I have a fairly flexible schedule and can get away with doing no work one day and a lot of work the next day. Nothing feels better than having all your homework for the entire rest of the week done on Monday.