There is one important thing when you are buying your groceries, rain checks. I know the idea sounds a little bit off, specially because they can only be acquired if the product you are looking is out of the store.
Yes, they can be annoying to get but at the same time they can be an important key when you stack old sales with new ones, the reason is - some of the sales you can stack with are money makers, depending on where you shop -. Shopping in a smart way can be quite a challenge, but when you organize your trips it shouldn't be an issue.
One neat trick about rain checks is that you can use old coupons (check your store policy, albeit is redundant because the importance of rain checks is to boost the sales and help the customer that couldn't) as well new ones that weren't at the moment of the sale.
I.e. if your rain check says you can get a Pantene shampoo for 2.00 when its regular price is 2.79 you will be paying only ¢.79 but, if during the sale, you get a coupon that you couldn't use and the time limit for the use was short, you will be able to use if it follows the same or similar parameters of the sale; so, if your rain check still active and you saw in the papers there are more coupons for the rain check, you can use them too if they match the sale parameters.
So, another trick to understand when shopping with rain checks is to always keep a list of the previous sales you could have, that were money makers because if your store accept amount discount coupons, mixed with the regular and in-store ones from previous sales, essentially the store will pay you to shop as you can mix several sales from four different weeks (as the time limit for most rain checks are 4 weeks or 30/31 days).
With rain checks you have to remember, the time is four weeks (one month), you can use coupons that are in-store coupons and manufactures, or competitors; but the coupon trinity rule change from store to store and you can use more than one rain check in order to maximize your savings.