Best advice i was given (credit to my aunt), when you're struggling to get into something just do 15 minutes work, then allow 15 minute procrastination as a reward, then 15 mins work again, another 15 min procrastination, keep going until the work's done or you stop needing a break. Usually after the first hour of this I've got my motivation back.

When I was at school I went from a C grade student to an A grade student with this technique.