Tip #1: Always keep a notepad and pen or dry/erase board on your fridge to use as a grocery list. That way, when you use up all the milk or eggs or pasta you have, you can write it down on your handy-dandy list, located conveniently next to you, staring at you in the face, on your fridge. And, if others use all the butter or cereal, they have a place to write down what is needed, too. When you and your family do this, you always know what you need to buy from the grocery store. You don't have to guess. And, before you go grocery shopping, make sure to take the list with you. Then, you only buy what you need.
****Keep in mind, this only works if everyone in your household participates. If your husband/boyfriend/significant other/child/roommate refuse to oblige with your new grocery list system, I advise you beat them senseless until they are willing to participate.