The only thing that has worked for me is Weight Watchers. I like their points system, it really helps me to make sure I don't over eat. Another one that is good (as long as you have self control, which I think for most of us that are overweight tend not to have that) is the South Beach Diet. Essentially what it comes down to is you have to change what you eat, how much you eat, and start exercising. It's basic and we hear it over and over, but for whatever reason it is easier said than done. If you can't join WW right now, you can try just the basics by getting what they call the good health guidlines in. Those are as follows:
5 servings of fruits and veggies (8 if you are nursing) - one serving is 1/2 cup unless it's leafy greens (like lettuce) and that is a cup.
Whole grains - as much as possible, make sure your bread says 100% whole grain, change to brown rice (if you can stand it).
2 servings of milk/dairy (3 if you are nursing) - a serving is one cup of milk, yogurt, etc. or 1 1/2 oz of cheese
6 8oz glasses of water (8 if you are nursing)
2tsp (3 if you are nursing) or healthy oils - olive oil, flaxseed oil, vegetable oil
1-2 servings of lean protiens - a serving is about 4 oz
Limit added sugar and alcohol
and try to take a good multivtamin (especially if you are nursing).
Exercise - try to do some form of exercise at least 5 days a week for 30 mins. You can simply start by walking. There is also a home work out dvd that I think works great, it's called "Power90" from beachbody.com.
Anyway, hope this helps and good luck!