I totally feel you, lol motivation? what's that?!
Anyway what I do is:
1. Get up, walk a bit and just stretch my legs etc
2. if I've been doing one subject for a while I change it up, vary it all I guess
3. Pretend I'm Elle Woods, after the party scene when "Watch me shine" starts playing and that I am FILLED WITH RAGE AND READY TO POUND MY RUDE LAW CLASS INTO THE DUST FIGHT ME
4. (Not my proudest feeling, but heck it motivates me) Also think about how I want to prove someone I know wrong and that I am justified in turning her down to revise and just generally make a point of doing better than her TO PROVE A POINT AGAINST HER ATTITUDE IM SO SICK OF IT AND I WANT TO WIN
5. Music can help if you've been working in the quiet, or if you've been listening to music then turn it off for a bit and work in quiet
Hope that helps and you get your mojo back! <3
good luck!!!