There is some good advice on this thread.
I work in the consulting/accounting industry that demands lots of hours sitting on my *****. Lots of my co-workers struggle to maintain their health/weight, there is always gimmick diets that lots try. Examples include: Atkins, southbeach, lemonade(that's all), low-carb, there is even a lady who only eats a "health cookie" 2 meals a day! In general, these folks see some positive results and rave to anybody who will listen about great the diet is. A couple of months later they back to square one.
I have maintained my bodyweight and general health since high-shcool by using things I learned in training for football. Eat-right, do cardio and lift weights.
Diet - starvation/deprivation diets with calorie intake too low can only last so long before you cheat. And diets that have cheating built in, don't make any sense to me. Here's what I do, I eat as much as I want but I don't eat things that I know are unhealthy. Cut out all fried foods, chips, and soft-drinks they provide nothing but sugar and simple carbs that your body immediately turns to sugar. Ask yourself what good is this food providing my body? protien, fiber, complex carbs.....vitamins....go for it. Drink water, lots of times when you feel hungry, you're actually just thirsty.
Exercise - I rotate 1 day cardio, 1 day lifting weights and take off if my schedule is too packed
Cardio - do whatever you like (run, walking briskly, mountian bike, swim, paddle) just keep you heart rate up for >20 mins I shoot for 30. Do it 3-4 times a week at about 60-70% of max. Something that yields great results is interval training where you go hard for 2 mins then go soft for 1 min. (so it total 2 hrs a week)
Lift weights - Here is an example of what I do, 3-4 days a week
Day 1 - Chest and legs
Bench - 4 sets, 10 reps
Squats - 4 sets, 10 reps
Dead-lift 4 sets, 10 reps
incline Bench - 4 sets 10 reps
Lunges - 4 sets, 15 reps
Push-ups - 4 sets, 30 reps
in total ~45 mins
Day 2 - Back and shoulders (after a day off for cardio)
Pull-ups - 4-5 sets as many as you can do
Military press - 4 sets, 10 reps
Rows - 4 sets, 10 reps
hang-cleans 4 sets, 10 reps
lateral raises - 4 sets, 12 reps
in total ~45 mins
lawn-mowers - 4 sets, 10 reps
There isn't one thing that is more important than another, you need to do it all!