I love the movies, and LOVE the popcorn so I usually, depending on how often I go, buy a small popcorn w/ no butter (they cook it in oil, so it really doesn't NEED the butter they put on there). It's generally around 350 calories for a small, and I work it into my daily intake so it's never a huge deal. And luckily movie popcorn seriously sits in my stomach foreeeever, so I stay full for quite a few hours.

I also happen to be a freak though, so I bring a massive purse and sneak in an entire two liter of diet pepsi, and usually an apple or a mandarin so I can have something sweet.

I air pop my own popcorn and smuggle it in. I have those popcorn shakers in butter, nacho cheese, white cheddar, and kettle corn flavors so it still tastes delicious but is much better for you than the movie theater crap.