Just stick to the basics on losing weight at any time and you'll be fine.
Drink plenty of water.
No bad snacks, only snack if you've quenched your thirst and waited 5 minutes (it really does help, plus if you drink water your mouth tastes neutral and the thought of adding anything sweet to it is quite off-putting).
All carbs, or as many as possible to be good crabs, not empty things like crisps and cake and white bread.
Three decent meals a day.
Plan your food, If I know I'm going to be eating cake at MiL's for example, I'd make sure everything else was good substantial food, if I am eating lasagne and jacket spud, I wouldn't try not to have any other carbs during the day etc.
Exercise, a small amount, just to get your metabolism kicking in, it can be chair exercises, doesn't have to be running or something, for me it's just getting one less bus a day and walking instead.
Dieting won't effect your milk, it may make you tired and hungry, but your milk will be fine.