After every purchase, put every receipt in a folder. If the item later goes on sale, you can take the receipt back and get a refund of the difference in price. Or if you need to return an item, (it breaks or you decide you don't need it or like it) you will be able to do so.
I don't organize them, I just know I can find it the receipt if I need it! After a few moths, I go through them and throw away the ones I'm sure I won't need.