I keep a running Wish List of things I'm out of, but don't necessarily need right away. Then I use it to make a grocery list and cross off the things I just need. That way I don't over-buy. I clip coupons and load e-coupons on my Safeway card. I read all the sales circulars to know where the best deal is. I get generic when it's really the cheapest (sometimes it's not!), get bulk dried beans and rice. I use leftover meat bones to make stock and freeze it. And keep a running Excel spreadsheet of what I buy and for how much so I can see what I have, when I bought it so it can be used up, and how much I've spent so far in the month. I also freeze a lot of leftovers with a label. And check out the discount meat bin at the store, it's everything that needs to be sold by that day and it's 30% or more off, and freeze it immediately.
Don't throw away leftovers. Seal any leftovers in a container then refrigerate. Then make Saturdays into leftover Saturdays when you can revamp the old dish. There are so many leftover recipe on the Internet. Also, buy in bulk for staple food items. You'll save more overtime.
Plan your meal in advance and list it down, buy in bulk you can save more when buying in bulk, use discount coupon, check the sale items and don't away the leftovers. You can make other recipe in some leftover. Have a grocery shopping list notebook and bring it with you when doing grocery shopping.