Before you checkout and even as you shop go to your preferred search engine and type in…
(Store name) Coupons
Up will appear websites that conveniently aggregate coupon codes for you.
You can sometimes click on a link and the coupon is automatically applied or they give you a code to use at check out. Also, there will be % success rate and expiration. Sometimes it will still work past the expiration date.
Why would someone put together a site like this? I believe they get a fee from the vendor if you buy something after using their link.
Anyways, I have saved a lot of money over time. Just remember though some coupons have a minimum like 10% off of $50. If you were only planning to spend $15 but decided to spend $35 more to hit $50 and save $5. Great. You may have just bought stuff you did not want or really need. Also you probably wasted an extra 30 minutes looking for stuff to add up to $50.
All in all as with everything buying something for less still costs money and time.