I've a history in dealing with customer service representatives and getting my way..
I can give you a few tips, the rest is up to you:
-Regular CS agents, most often, can't do shit. Unless it's the company's policy to refund or such (and if that was the case, you wouldn't need to be persuasive).
Don't waste your time with them, because there's nothing they can do and besides they don't give a shit: whether you're happy or not, they'll still get their salary at the end of the month, they're not responsible for sales and company incomes.
-Try to get a supervisor on the phone (or write to them). But don't go demanding to have a supervisor on the line and don't be arrogant. If necessary, call multiple times until you find "a sucker" who will get you a supervisor on the phone, or will give you a number/email address.
-If at all possible, talk to them on the phone, DON'T mail.. it is much easier to persuade someone on the phone than in writing. Plus, on the phone you can be anyone you want to be.
NOTE: I'm NOT saying you should pretend to be someone else. I'm just saying that on the phone you can easily sound self-confident, well mannered and "important", even if you're a pathetic little nerd in rl.
Of course it's much harder to do this if you are 14yrs old and you sound like a little kid, because most people won't take you seriously.
-Like Smoobies said, "butter them up" first: these folks hear complaints everyday and they immediately shut the door on your face if you are arrogant, shout etc.
Instead, you should let them know that you are truly disappointed (not pissed), because you've been purchasing sony stuff your whole life. you had psx, ps2, you have the ps3 and your son (or brother) just got a psp for his birthday, you (or your father) bought a 42" Bravia just last christmas and for the first time you're really disappointed.
DON'T ask for a discount or such, at most you can hint that that would please you (though I don't see how they could arrange a discount over the phone, but it's still worth a try). It is important that THEY offer whatever bonus to you.
Among the things I've done, I've got huge discounts, entrance to clubs skpping the line when I was a teenager, managed to exchange stuff months after purchasing it and without the receipt, sometimes even with the new model (like ray-ban glasses lol), got in for free and without invitation at various concerts, WWE wrestling, the italian version of the US emmy awards (multiple times) and so on, so I think I know a thing or two