Eating the correct portion size is a difficult task...I have trouble with this too! It's tough when you have gone hours without eating (because your simply too busy?) and once you finally get to eat you just want to PIG OUT! Unfortunately, this is a big reason we pack on the lbs.
How do I overcome my personal struggles with portion distortion? I like to pack a lot of snacks; right now clementines are GREAT, their small and fit in just about any bag you've got! I also keep my water bottle close so if I start getting hungry I sip some water (sometimes signs of thirst can feel like hunger) and when it comes to dinner time, I split my dinner in half. Eat half and doggy bag the rest for lunch the next day!
What do you do to overcome portion problems?