Physical Fitness Exercise Programs

First thing’s first, i’m 16 and 5ft tall. I’ve always had a bit of a horrible figure, and for about 3 years i’ve been on and off dieting. Mostly weight watchers diet and cutting out junk. I started at 9st and my lowest was 7st 10lbs.
Then I really decided to kick start my diet again this summer, so i could fit into a bikini, and have now got down to 6st 9lbs as of this morning.
I’m actually happy with my weight now and want to maintain it, but i’m unsure what to change about my diet to do so. At the moment i know i’m not eating enough (probably 500kcal a day, if that!) and i’m burning about 800-1000 kcal on the treadmill most days. (although it’s really less because the treadmill’s programmed for a man!).
So basically i know i need to eat more and do less exercise to maintain…. but i have no idea how to do that gradually… if i go up to 1500kcal over night i’ll just pile on the pounds, ditto for cutting back a lot on the exercise…. any tips from people who have come off a diet successfully?
I also want to know a good contingency plan, so if i want a slice of cake of a chocolate bar one day, i know what to do to burn it off and not gain a pound without feeling guilty!

Try to go up to around 1000 calories, weigh yourself, and in about a week, see if you’ve gained/lost/mantained your weight. Base your next step on the results.