I'd say that the biggest waste of money to avoid is by being a BRAND Snob! Do not rely on certain brands. Do not only buy a brand you think your family loves. I'm not saying to purchase the store brand by any means, or the cheapest brand. Different brands will have sales and coupons at different times. This means that even the most expensive brand may be free or close to it at a certain time. PLUS, you may also find that other brands taste better than you thought.
It really is important to not only focus on the brands you're use to. Once you start getting the brands that are on sale or have coupons, you will really save a LOT MORE MONEY!