Record everything you spend. This is just like calorie counting: the mere act of paying attention and recording makes you spend less and helps you notice trends in what you are spending. It also really helps you feel confident and in control.
My husband and I maintain a monthly spreadsheet where we itemize every individual item we buy, where we bought it, and what we paid for it. Just doing that has saved us a ton.
Once you have a months' worth of records, figure out where you spend the most money and find ways to cut that. If you go through a jar of peanut butter every 3 months, it doesn't really matter if you pay $2 or $2.50 a jar. But if, like me, you eat over a pound of cottage cheese a day,you really want to pay the best possible price.
One more thing: don't hoard food. Eat whatever is in your pantry, and once you have it empty, keep it empty, buying just what you need for the week. Food hoarding is an expensive habit because it keeps accumulating--without realizing it, you keep buying things you don't intend to use any time soon, and sometimes you keep buying the same thing over and over because you don't realize what you have.
Part of not hoarding is committing to going to the grocery store as often as needed. When it was the "rule" that I only went once a week, I tended to overbuy "just in case". Now, I buy exactly what I think I need and when tempted to buy more, I remind myself 'I can come back'.