"How do you stay so thin?"

I was at the gym when some girl asked me that.  It kind of caught me off guard too.  Well, I wish I had some big secret to share but I don't.  It's a bit of genes but mainly it's 'calories in vs calories out'.  Everyone in my family is fairly slender, both immediate and extended.  A high metabolism must run in the family.
That doesn't mean if you don't have a high metabolism or your family isn't generally slender, you're screwed.  The biggest thing is the number of calories you eat vs the number of calories you burn.  I'm sure you watch what you eat and you go to the gym everyday and sweat like crazy while running on the treadmill for an hour and a half, but if you can't lose those last 10 lbs, or you don't see a difference in your weight, then I'm also sure your watching yourself eat that quick slice of pizza after class/work or that Big Mac from McDonalds with large fries and a diet coke that you only eat once or twice a month.  You up-ed your intensity/duration/weights today at the gym, so you can treat yourself to a pasta dish with a creamy sauce you need to replenish the carbs right?  do you really NEED to?? You ate a healthy salad and an apple for lunch so you're allowed to have some chips or cookies after dinner.  You'll be going to the gym tomorrow anyways...
Do you see where I'm going with this?  Those little exceptions that you make for yourself add up!!  Once or twice a week is enough to keep those last few pounds from melting off.  
I'm not saying stop eating, I'm not saying workout longer and I'm not saying, forget about the gym cause you're gonna eat that 99 cent pizza anyways.  I'm saying refrain from 'treating' yourself with bad foods and eat healthier while keeping that work-out intensity up.
Don't succumb to the McDonalds drive through because you're hungry and it's on the way home from work.  Keep a bag of nuts and dried fruit in your car to chomp on until you can get home and make yourself a proper meal.
OR!! stop complaining and whining about your fat thighs, flabby arms and big stomach come bikini time or summer time for you guys and just enjoy every last bite of your Double Quarter Pounder with bacon and cheese.