Tip #1
Consistently buy groceries from places that make you pack your own groceries and who charge for plastic bags. Typically, items here are much cheaper than at other chains.
Tip #2
My brother is finicky about the brand of his ketchup and cola. Unless your finicky about brand, go for the store brand. Personally, most store brands are indistinguishable from name brands. The markup you'd pay is almost always solely for the name.
Tip #3
If going to the corner or convenience store is so much of a happy experience that you don't mind paying for it in double the prices on grocery items, go right ahead. For most of us, convenience stores are where those who waited for something to be urgent end up getting fined.