So before you go to your first store armed with a list and some coupons, I want to offer you a few suggestions.
Prepare! Write down what you intend to buy, the coupons you will use, and the expected cost. Why? Sometimes I get so excited I forget what I intended to buy, and you will to. Sometimes a coupon will fall into the cart or your 2 year old will truly believe that is her prize for good behavior (okay maybe that's just my child) but what I'm saying is sometimes the coupon you need got misplaced and you need to know that you even had that coupon to use to begin with. Heaven forbid - the cashier rings something up incorrectly (okay this happens often). You should know what you intended to pay and question it if it isn't what you expected. Double check your receipt when you get to the car and ensure everything looks as you intended. I've personally had clearance items not ring up, coupons not scanned, and things that were marked on sale not ring up on sale at all. If I wasn't aware of my intended spending I would have just gone with it.
Bring all your coupons - everywhere you go! Sometimes you get to a store with your intended shopping list, and all the other gosh darn couponers got there first. The items you intended to buy are no longer on the shelf and although you can get a rain check for the sale item your purchase no longer adds up to $20 to use the Rite Aid $5 off $20 coupon. Or you are walking around and find your favorite item on clearance. In the first scenario if you have your coupons you can insert other items on sale to make up for the item that is no longer available to reach the necessary $20 and in the second scenario it is awesome to find clearance and then pair it with a coupon you so thankfully brought with you. Not having your coupons when you find a steal is a big bummer so its easiest (and most cost effective) to take them everywhere you go shopping.
Be an advocate for yourself! If you know the coupons you have are correct and the cashier is doubting your transaction, ask for a manager. If they are still insisting you are wrong, ask for corporates phone number and call them. Yes you read that right, go to the big wigs with couponing questions. I've done it myself and the manager might not have been too happy with me but it was a big difference in savings (and lo and behold I was correct, thank you!).