I'm pretty organised. Here are my top tips:
1) Diet-wise, the best advice I can give you is to try and eat your five portions of fruit and veg before you eat anything unhealthy that day. E.g., chop up some banana to put on toast (with something like nutella, honey or peanut butter) and have a smoothie for breakfast, snack on satsumas throughout the morning then at lunch eat a sandwich with lots of salad (or just the salad on its own) plus some carrot sticks dipped in hoummus. That is your five a day wiped out by the middle of the day and if you can eat some veg with dinner, so much the better. You'll feel so much better knowing you've already gotten your vitamins, minerals and fibre and you will also feel much less tempted to snack on unhealthy stuff because you'll feel more full, and you won't want to undo your hard work.
2) With exercise, there's no point trying to stick to a routine if you're hopeless at keeping to it. Try and have an exercise quota that you fulfil every week rather than every day, for instance I try and go to the gym at least four times a week, but I don't have set days when I go because this gives me the flexibility to allow for days when I'm upset or tired, or if my friends decide they want to go out one evening. I don't bother going on the days when I feel really wiped out because I know I'll just do 20 minutes then give up. I'd rather get an early night and go another day when I know I'll do at least an hour.
3) I think we can all sympathise with the feeling of not having done enough school work. Right now I am meant to be writing up a lecture on skin function but TSR just seems so much more appealing (and yes, thanks, I am aware of the glaring irony that I am sharing my tips for an organised life). The only way I can find to avoid the computer and the TV is just not to turn them on. Everyone kids themselves they'll only spend half an hour surfing the net, or will just watch Scrubs then they'll turn the TV off but the reality is there is always a new post to read on TSR, a funny website to look at, someone else logging in to msn, a suddenly intriguing storyline on Hollyoaks just after Scrubs finishes etc etc. These things are the biggest time-eaters so just don't go near them until you have done at least enough work to salve your conscience a little. I try (haha) to have a rule that I don't turn the telly or the computer on til 9 in the evening, by which time I'm usually too bored of being good to be bothered with much more work, and I'm not constantly thinking "oh just half an hour more, then I'll do my work" - it's guilt free leisure time. Try asking your friends not to call or text you til this time as well so that you don't get into interesting conversations and waste the evening.
4) The best tip I can give anyone on being organised about anything is to make lists. I am the list monster. I make lists for everything - things I need to do, things I need to buy, work I need to finish, things I feel happy about, things I feel sad about, the list goes on. No pun intended. Every day write down a list of things you want to accomplish. If they are there sitting on the page in front of you, you won't forget anything, you're more likely to feel obliged to actually do them and you'll also feel more in control and proud of yourself when you can go over them at the end of each day and tick off everything you managed to finish. If there's still stuff left, just carry it over to the next day.
5) Lastly, don't go mad if you have one bad day or even a bad week. Sometimes everyone feels so unmotivated that they eat crap, do no exercise and finish no work. You're only human and it happens to the best. Just don't let it become habit - if you have a week where you've done little else but watch reruns of Friends, don't beat yourself up over it but resolve to do much better the following week. If you really can't monitor your own behaviour then it's worth giving your lists to someone who will give you a kick up the arse. I find mothers are particularly good at this; my own takes a great deal of smug pleasure in using the phrase "shouldn't you be revising?" at the most irritating moments, i.e. when I have just finished a stint of hard work and have taken two minutes out to put the kettle on.
I really hope this helps because I would like to remind you once again it was at the expense of writing up a lecture on skin, the most interesting organ ever. And for that I am eternally grateful.