One thing that is TOTALLY pointless is "going on a diet"

The only way to lose weight and actually keep it off is a lifestyle change.

I lost 1.5 stone in 6 weeks a couple of years ago because I decided to see how much weight I could lose in 6 weeks just for something to do (was exercising a lot more too as well as the diet), but because at the end of that period I then went back to eating and (not) exercising exactly as per before, I am predictably now exactly the same weight again as I was before the diet/exercise regime.