My first couple of Costco trips went a little overboard. Here are some tips that helped:
1: Don't buy something in bulk unless you've used at least that much in regular quantities.
The only way to know for sure if you're the kind of person to finish a 50lb bag of rice is to already have eaten 50lbs of rice.
I like this rule because it's self adjusting.
2: Choose an impulse buy budget before hand
When I shop, I give myself permission to buy 2 food and 2 non-food items on a whim. Everything else must be on my list. This is much easier to stick to than refusing to buy anything on impulse.
3: If you only need a few items, carry them.
Walking around with an enormous cart kind of compels you to fill it. If you only need a couple items, carry them by hand. #finance source