Twiglett - I got this book which I found really useful. I think if I'd stuck to it I might have lost a bit of weight, but I wasn't disciplined enough.
It's not a diet - it's a healthy eating plan with guidelines for filling foods with the correct amount of calories needed to support b/f which are suitable whilst feeding (rather than the rubbish that I reached for when I was unfeasibly hungry due to b/f).
It's written by a nutritionist who seems to talk a lot of common sense - especially about what you can and shouldn't eat whilst feeding. Nothing seems to be forbidden, but there's a fascinating chapter on caffeine.
Eat Well and lose Weight While Breastfeeding
Weightwatchers (quite rightly) won't accept a b/f mum until baby is 6 months old and then you need to go to the meetings to have a special plan devised for you.
hth