In my healthy way class I just finished, this was stressed. Chocolate milk is a great post-workout snack if you don't want food.
We used MyFitnessPal (free website) to track our food and exercise. The website sets your calorie goal for the day, taking into acct how much weight you want lose per week (.5 to 2 lbs per week). Any exercise you do, you add that number of calories to that number.
Per the nutritionist: Most people are not eating enough to lose weight. They think they need to stick to 1200 calories plus exercise. That is not enough to eat. Say you eat 1200 calories and burn off 400 calories swimming. That is net calories of 700 for the day. Heck, your brain alone needs 600 calories a day to operate. That only leaves 100 calories for the whole rest of your body to run! It's easier for your body to burn muscle than fat, so it will start eating at your muscles for those extra calories it needs, leaving you all flabby and not losing weight.
After 2 years of trying and trying to lose and doing nothing but gaining, in my 12 week class I finally lost 20 lbs following this advice. My net calorie goal was 1650 calories. So if I exercised 400 calories off, I could have up to 2050 calories that day (which still works out to 1650 net, 2050-400). I usually didn't eat that many, but I made sure my net for the day was at least 1200.
I still have at least 70 more to go, but it's a good start!