What's always worked best for me was to nearly be an a** ***e about buying. The last bass I bought off the shelf was my IbanezBTB406. Sticker said $695.
This was (basically) my conversation with the guy at Sam Ash.
Him: "So.. you wanna take this one home today? (fake salesman laugh)"
Me: "Not for $700, you're out of your mind if you think I'm paying that much for something that I know has been sitting on you wall for two months getting played by every hack that's walked back here. What can you do with it?"
Him: after a blank stare for a second or two "Hang on a sec. I'll go look it up for ya".....
.... "My manager said I can drop it as low as $625, but that's it, it's the last one we've got and don't know when we'll bet another in"
(Not that, that had ANYTHING to do with selling this one.)
Me: "You're kidding right? I'd give you like.. $500, Maybe $525 for it."
Him: "Na.. I can't go that low on it man. But $625 is a steal on this thing.. it lists at over $900"
Me: "Nope. Forget it."
Walked to pro audio, then slowly wandered back over to the bass section where I was soon approached by the guy and his manager. Explained that I wasn't going that high on something that was now, to me, a used instrument, since it had been there for a long time and been played by so many people and misshandled.
Ended up walking out with it for about $550 with a new set of strings thrown in, and a SKB road case for about $50.
Don't be afraid to be a loud, annoying, @$$ bag. Just don't over-do it to the point that you get kicked out.
Then throw in a little "I really don't care if I have this bass or not. I was just wasting some time today" and they'll start dropping the price with little hagling on your end.
When all is said and done, you'll work them over like a cheap hooker, and at the same time make them think THEY were the best salesmen Ever, because they were able to talk someone like you into buying!
Suckers.