Start checking grocery store tags to see how much an item costs per ounce, per pound or by some other unit of measure. Comparison-shop by unit price and save.
A pack of 40 diapers at our local drug store cost $13, or 33 cents per diaper. A box of 144 diapers cost $35, or 24 cents per diaper. A difference of 9 cents may not seem like much, but when you change a diaper six to eight times each day, that amounts to a savings of $16 to $22 per month.
Don't buy in bulk if you won't use it all -- otherwise, you wasted your money, no matter how good a deal it was.