Grocery Planning at our house starts with the meal plan. I have a form I use to plan out our meals. Then I write down everything we don't have in storage that is required for those meals. I check and double check our pantry, fridge and storage for items we need or are running low on. Then I evaluate my list and decide what is really a need and what we can do without till next week. I also estimate how much the items on the list is going to cost me, and adjust accordingly.
My grocery shopping list is a form I print on the back of last week's menu plan. I have categories organized in the same order as I move through my favorite store. This keeps me from running all over the store looking for things I've forgotten. The quicker I get out of the store the smaller the chance I will make an impulse buy.
We also have a notepad that lives on our refrigerator. The notepad is where anyone in the house can write things we would like to buy. It doesn't matter if it is a need or a desire, we write it on the list. Special treats, fancy electronics, stuff for the house, it all goes on the fridge list. When I have left over money in my grocery budget we can get things on this list. And, it is a nice place to find birthday ideas.
Using a list ensures that I'm not running to the store a number of times during the week to get things I've forgotten. It also helps me to stay focused during my shopping trips. All of which helps me spend less.