Not that I'm an expert at this or anything, but I'd suggest:
Don't set up a crazy-hardcore plan or promise to work out at the gym for 3 hours a day, every day, because you're probably not going to stick with it, and instead of just doing less, you'll end up doing nothing at all. It's best to find something you like to do and make time to do it a few times a week.
Same with diets. Those extreme diets rarely work and it's better to just make little changes to eat healthier than to cut out certain foods completely.
To stay in shape, the little things work for me. Like...if I'm watching TV I'll do crunches or little exercises while I watch. When it's nice out I try to jog once or twice a week. For me, it helps that I can't stand soda and pretty much only drink water or juice.