I always pitch at 10% less of the sale price of whatever it is (only with guitars or amps though - too much effort on lower cost items). Like the others say, just tell the guy what you'd pay (not what you can afford), and then walk away if he doesn't go for it. If you haven't already got it for 10% less, he'll want to negotiate as you're heading out the door.

Another trick I've used is to go on about a minor point - say for instance the guitar doesn't come with a case - just keep on moaning about how it hasn't got a case, and use that to start bending the price down. Missing Trem arms are another common one in the places I go.... £10 for a new one but potentially way more just 'cos someone nicked it in the shop.

Retailers will sometimes match online prices, and some do 'pricematch' as well. Make sure your offer is realistic though - you need credibility, which you won't have if you offer the bloke £25 for a PRS custom 24... 10% works for me though.

Next idea is to keep going back in to the same shop over a period of time and see what ISN'T selling. Making offers on goods the shop cannot shift can be even easier - I got a very nice US Tele like that.... :D