Another disaster grocery shopping

Not only is our one night at the water park costing us $360, last night we went to the grocery store to get food for the trip. My guy and I talked about it and decided we would only eat out once and the rest of the time we could make something in the room. There is a microwave and frig so off to the grocery store we went. Since I've been lazy and hadn't looked/cut coupons in a while, we just went to Winco. We had a game plan to get sandwich stuff, chips, easy mac n'cheese and popcorn. Once again, $76 later we were on the way home trying to figure out how this ONE trip is costing so much more than we planned. Did you know that when you buy sandwich stuff you need to get: cheese, mayo, meat (2 different kinds) and bread. Now when you add that all up it's only $10 which is much more reasonable than going out to eat; but since I was at the store I decided to pick up some soup for lunch and more yeast and about 50 other things I probably already had or didn't really need, like the chocolate cover banana chips that I'm currently eating!

How do you walk into a grocery store with a list and not buy anything else?

Right now our Christmas present to L is at $434, plus I will need to put gas in my car and I'm sure that stop at the gas station will result in some snacks for the road. Now, I've never been one to deny L anything and normally when we stop for gas/groceries/Wal-Mart and she asks for a snack/drink/gadget at the check out its no big deal; really whats another $1 ... but now that I'm (we're) trying to save and watch spending, a dollar adds up quick. How do you teach kids that a dollar at every stop adds up to $10 by the end of the weekend and we just can't afford it anymore.