Have you tried counting calories? I have found that to be a great way to lose weight - I didn't "think" I was eating as much as I was. Once I started counting, my portions got MUCH smaller, but after a couple days of headaches (getting used to not eating my normal amount) now I feel great (I started doing this again last Sunday). In only 8 days I can already see some improvement!
I did it when DS was an infant, about 5 months or so, and I got to pre-pg weight. However, since then I've gained 15 pounds (which is why I'm trying to lose again now). I don't (in my case) feel it had anything to do with nursing itself but just more with the whole "mommy" lifestyle and not taking time for myself to pay attention to what I was eating and making time for exercise.
I've heard people say they weaned so they could lose weight, but in those cases I think it was because they wanted to take some of the drugs on the market now that help you lose (metabolife, etc.)