hmmmmm, motivation.
years 7-10 just cruised through, but i did pick up a bit around the school certificate exams.
year 11: started working properly. did homework in all my free periods, as much work and study as possible before dinner, that sort of thing.
year 12: more of the same, but less intense, i suppose i was burnt out or something... but it wasn't too bad. take it easy, don't drop everything for the sake of study (but by the same standards, now wouldn't be the best time to pick up another shift at work or join 3 new sport teams
), and just take stuff a few things at a time: "ok, i have a chemistry assesment next wednesay, an art essay due on friday, and a maths topic test next thursday" -- i broke stuff up into little chunks of a few weeks each (but towards exams, look ahead and study in advance)
that's all i can think of at the moment... it didn't always work for me, eg at one stage i was seriously demotivated in art for 2-3 weeks.