Eat less - excercise more.
That's about it. Oh, and getting rid of high fat, high sugar food and
replacing that with fruit and veggies helps too.

Try a high fiber diet--it works. I use Brenda Watson's book Fiber35
but basically, just make sure you get at least 35 grams of fiber a day
and plenty of water.  Veges and fruit have lots of fiber and so does
ground flax seed. I also use sprouted grain organic bread (Trader
Joes) and it is very low in carbs and high in fiber and wonderful
toasted!

Anything that lowers insulin response (blood sugar) will lower body
weight.  Don't count fat grams, carb grams, or sugar grams, it isn't
necessary.  Instead, make your focus the *quality* of fats, carbs, and
sugars.  Foods with a GI under 65 are best, so you do need to be
familiar with which foods fall where.  Your average food GI intake
should be somewhere below "60."   A banana for desert each day (GI-51)
will act as a weight-reducing food when compared to any processed
carbohydrate snack.  Unsweetened juices are perfectly Ok, as well.
Choose metabolic-friendly fats, such as virgin coconut oil, which can
be used for all your cooking.  For salads, use either virgin cold-
pressed olive oil or lightly refined walnut oil (I use Spectrum brand
for its more neutral flavor) or you can also use deordized (flavor
free) coconut oil.  Saturated plant oils are not metabolized like
saturated animal fats, so use all the organic coconut oil and eat all
the avocados you like, but limit red meat (assuming you like it) to
once per week.  Choose sweet potatoes over white potatoes, choose
brown rice or red beans instead of white rice, choose sprouted wheat
over non-sprouted breads, and replace coffee and soda with unsweetened
tea (I use stevia and xylitol together.)  Avoid all packaged crackers,
though rye crisbread can be used as a compliment to soup.  Soups
should be organic and trans fat free.  With this system, you can eat
all you like and you will never have to count calories, carb, or fat
grams.  I would walk at least 1 mile per day at a good clip and take a
quality whole-food vitamin from your local health food store.  [ref.
http://www.carbs-information.com/glycemic-index.htm.]