What are some good organization and time management strategies?

I seem to be completely disorganized when it comes to my cles and ignments. I start the school year being so organized that i actually overachieve and get As on everything but then i steadily descend into a period of slacking off and laziness. I have no idea why. How can I make it easier to become organized and become motivated? I recently failed a super tough honors cl and now i have to attend summer school! Thats just not me. I really need some help and advice. Thx.