I think daily exercises and workout in fresh air is very important.
Including proper diet, never ever try to be hungry for being slim. Whenever you feel hungry eat something, because the research says that being hungry increases belly when you intake food after long fasting.
Also try not to take much oily eatables.
There could be many more ideas definitely, but these are the ways which I myself is following. I can understand your concern because I too gained a lot weight after my pregnancy.