Procrastination is the worst disease a year 12 student can catch. However, if we're brutally honest I think we'd all have to say that it effects us a some point. However, I can be overcome! This isn't from a scientific study or anything, but it does wonders to my study style and gives me a boost. When I get unmotivated, when I am bludging around or when I am not studying competently, I either lay into my boxing bag, or do as many push-ups and chin-ups as I can. What this does psychologically I have no idea, but it does take your mind off school for a bit, allowing you to get your head back in order!
Sorry if this doesn't help, but it might work!