There is no guaranteed way to do so - else we'd all be doing it. Gambling, stealing, dealing all might yeild some cash, but your best bet is to realise you can't "make a bit of money quick". How much are we talking about anyway?
Car boot sale? You wouldn't believe the tosh people buy. I made �350 from the first one I did. Though it does mean getting up early on a Sunday and finding a trestle table from somewhere, it's actually quite enjoyable once you get into the swing of things!
telemarketing- you have to be strong when people tell you to p*** off but you don't need many qualifications. I know it's an extra job but my frineds did it to pay off their student loans and they're very happy! Or you could try to SAVE money like buy recycled toilet tissue and kitchen roll or buy supermarket brand stuff you know- little things.
Some people make good money by sending off to every competition and prize crossword they can find, although there is no assurance that you will win...or if the competition is even legitimate.