This is going to sound strangely basic, but most of us don't keep our receipts for very long. I learned the hard way several years ago to keep my receipts when I couldn't return a defective item for refund because I had thrown away the receipt.
When I order online I now limit myself to sites that provide free returns. This means that they pay the shipping if you decide to return your purchase. Keep the original box and the shipping materials for a couple of weeks too.
Most stores (Target was the first I knew of) will reimburse you for the difference in price if an item that you bought goes on sale within two weeks of the original purchase date. Bring your receipt in to Customer Service and they will scan it, then reimburse you either with store credit, credit card credit or cash. My Target will just hand you a cash reimbursement.
Also store the return label for your online purchases with the receipt.
There is no reason to keep something you don't like, especially when you've spent your hard earned money on it. And, yes, many places will allow you to return something you have used if it turns out to be defective or not work for you (how are you going to know if you don't use it). But please be ethical about it; don't buy something to wear for a special occasion, then return it when you are done. Tacky and this type of abuse leads stores to change their policies, which hurts all of us.