The midwife I was seeing for both of my pregnancies really emphasised healthy eating and for the most part gave me really good advice. However, when my first (ds) was born she was pushing me to eat big meals all the time, so that I would have enough milk. As a result, I ate when I wasn't even hungry and it took me two years to lose all the weight (he nursed until age 2 1/2)

:. With my second (dd), I wasn't really that hungry after she was born; I only ate when I felt like it and within about 14 months I was back to the same weight as before my first pregnancy. She is 18 months old and still nursing.
I think that maybe some of the people who are having trouble losing weight while nursing unconsciously believe that they have to eat more in order to produce milk. Maybe it would help to keep in mind that excercise actually increases milk production, while eating more probably doesn't.
Another thing: I think that some moms are so focused on creating a routine for their kids at home that they spend too much time at home during the day, which invariably leads to more eating. My advice is: at least three times a week, pack some healthy snacks (nuts, raw veggies, fruit, cheese) for you and the toddler, and spend the entire day at the museum/ park/ botanical gardens/ library... you get the idea. Carry your baby in a sling or backpack, or push them in a stroller; whatever works for you. just get out of the house and don't come back until the evening I find that when I am out doing things I tend to munch less, and will only stop and eat when forced by hunger, which is pretty much the way humans were designed (or have evolved) to act, I think. It also teaches the kids to be flexible and to adapt to new environments.
If you are not convinced, think about all the hippie style moms you have seen wandering around at the natural history museum or the aquarium with their babies in a sling. Most of them look pretty healthy right?
Good luck!