Those with long memories and a habit of reading Savygamer might remember when 10% off Zavvi coupons were being given away in return for recommending friends to a product (Walkers rainy day something IIRC). Anyway, they realised after the word got around what had happened, and sent vaguely threatening messages to LewieP about exploiting loopholes, and presumably someone in Walkers giveaway team got a kicking.
Anyway, Zavvi have now got involved again in a similarly exploitable campaign, and Lewie's mentioned it again as well. Decent instructions are
here, but it's basically going
here and supplying any details you fancy (albeit a real UK address, but not necessarily yours), then making up random names and emails (I used the Beatles and the Chuckle brothers) and 'recommending' them. Each recommendation gets you a point, and 3 points gets you 15% off at Zavvi (and IWOOT, LoveFilm and buyagift, if they're more your sort of thing). I used it to preorder Brink, Shogun 2, and Ass Cred 2: Bros in da hood for roughly £20 each, which is a pretty good premium games haul for one evening. It can be used on any current thing in the Zavvi store under £100.
With pre-orders, you don't pay a thing until it dispatches and you don't pay any more than you see at the time, so you can always change your mind. It's not quite as good as Amazon in that it doesn't track the lowest price, but it's much cheaper and I doubt you'll get lower than that for most games before release.
(On the topic of pre-orders, people interested in Diablo 3 should definitely look at
it on Amazon; pretty much guaranteed it'll shoot up to £35 on release, and not be that low for a long long time after)