Start by repeating to yourself the following: "They want to give me my money back in exchange for returning this horrible piece of crap. I need only remove the obstacles they may feel duty bound to put in my way."
Repackage the item so that it looks as identical as possible to the way it appeared on the shelf. Store managers love that, and it's the reason I hang onto all packaging for many weeks. Present the package and your receipt to customer service with a plausible explanation of the problem. Try to make it as technical as possible, and speak in a monotone. ("The thing only runs off the USB port, and my PC doesn't have one.")
Be aware of the store's return policy. If you're not supposed to get a cash refund after 30 days and you waited for more than a month to take back the thing, practice your look of astonishment in the mirror. Then play it like this:
Service Rep: "I'm sorry, but all we can offer is an exchange for another item."
You, jaw dropping: "What? What!? I can't get my money back? You sell me something like this and I'm supposed to trust that some other product in here is gonna work?"
At which point the clerk either will relent or dig in the heels and get even more obstinate. Time to switch gears: "You know, the consumer protection laws in this state supersede the tiny print on the wall back there. I'm entitled to a full 90 days to get my money back. Now, I don't want this to get ugly, and I sure don't want to dump on you, but the law's the law."
Try to do a difficult return like this as close to closing time as possible, so the clerk will be motivated by the going-home imperative. Definitely do it after midnight at a 24-hour store.
It is even possible to return things you didn't buy at a store through the simple expedient of making up a phony cash register receipt. This requires that you keep a supply of register receipts from various emporia, noting the coding and printing (dot-matrix, inkjet). These you can duplicate on your computer. Unfortunately, many transactions now are tied by transaction number to the store's computer network ... but keep in mind that sales clerks are quick to point the finger of blame at the computer and not at you. Home Depot and Wal-Mart employees are continually pissed off at their stores' computers.
Wal-Mart, by the way, will take your stuff without a receipt as long as it's stuff likely to be found in Wal-Mart. Even if not, you can claim another store of origin. I unloaded a useless rug shampooer (in-law gift) at the Gloversville Wal-Mart simply by insisting I'd bought it at the Albany store and lost my receipt.
Sometimes the return threatens to be a huge challenge. I was stupid enough to let a malfunctioning snowblower sit in a backyard shed throughout the winter. I used it once, after which it never started -- and it ended up blocked into my tiny backyard. So it wasn't until April that I boxed up a December purchase and headed for Montgomery Ward.
My wife and I prepared for the occasion by dressing in farmer-style clothes (plaid shirts, overalls) and affecting back-country accents. And we planned a good-cop-bad-cop scenario, that played out like so as we stood in the crowded customer-service area:
Me: Got a snowblower to take back.
Clerk: Can I see your receipt? You bought this in December! I can't take this back!
Me: It don't work! I spent six hunnerd dollars on this an' it don't work! 'Sbeen stuck in my backyard all winter!
Clerk: I'm sorry, I can't --
Me: I saved up six hunnerd dollars t'buy this, and you're tryin' to rob me o' my money!? (Addressing the crowd:) That's what they do, yuh know! They rob you! They take your money away! Goddammit, they're STEALING FROM ME!
(The crowd started murmuring in assent. A wonderfully ugly mood hit the air.)
My Wife: Honey, don't get this way! Calm down! (To the clerk:) I'm sorry, but he gets this --
Me: DON'T TELL ME HOW TO BEHAVE! THEY AIN'T GONNA STEAL MY MONEY!
My Wife: Please, dear --
(Then I threw in the magic phrase, that gets your adversary instantly off the hook:)
Me: Does this have to be your decision?
The clerk shot into action, fetching a manager from the bowels of the store. I heard the manager ask, "Does the snowblower look OK?" and the clerk answered, "It looks new." Said the manager: "Then give him his money back."
We claim no responsibility for the store being out of business.