I'd try the Master-Cleanse -- everytime I do it I lose quite a lot.
You have to be prepared for a little suffering - it's not easy.
But you'll definitely lose weight F-A-S-T! :-)
And not to dispute any other suggestions that have been given, but I did the Atkins Diet (strictly) for 8 weeks and lost only 7 lbs... I called the helpline and spoke with the "Atkins people" and everything -- just didn't work for me.
However when I finally decided to try a vegetarian diet I lost 13 pounds in a little under 2 months.
But for quick loss the Master-Cleanse can't be beat - I've read amazing numbers and I always lose between one and two pounds a day.
And if you are doing it for weightloss and get the added bonus of a cleansing, well, then that's just what it is - a bonus!