h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If you exercise for an average of about hour or two a day, you won't really need to bother with a diet, just avoid fast food and make sure to eat plenty of good, healthy stuff. My sister is quite slim and her friends can't understand how with all the food she eats. The answer - excercise! Excercise will make you gain weight, but that is to be expected - fat is lighter than muscle.

Excercise most people can do:A brisk walk, running, cycling, swimming.
Rowing is absolutely excellent, but not many people get the oppertunity. When I joined my school's rowing club, I must have burned off several kilograms of fat and replaced them with muscle.

Some good foods to complement the excercise: Most past and rice dishes (preferably homemade) red meat (yes, it's good for you), white meat, fresh salad (again, homemade, best prepared just a few minutes before the meal), potatoes (surprisingly versatile, but don't bother with fries and chips/crisps), bread (which can be livened up by putting stuff in the middle), fish (though is low in protein).

Some nice and healthy stuff to put in sandwiches: Cheese, ham, chicken, tuna, lettuce, tomato.

Correction, fish is high in protein, around 25g of protein per 100g basically the same as meat.

Oh, right, woopsie. One other thing - don't skip breakfast, I did once and I didn't feel right all day. It needn't be anything fancy, just a bowl of cereal and a cup of water is all I have, though obviously avoid cereals which should be classified as chocolate, candy, or sugar.