wanna bargain? can, but know your stuff first.
firstly if you know your stuff well, done your research, you roughly know how much the item you're looking for costs, and then you can roughly gauge how much discounts you are entitled to, or rather you can get.
secondly, always be polite and never rude, and as the buyer, do the speaking yourself, never let a third party ( your mom, dad or something ) interfere as normally the tone they strike across is disgusting and would ultimately not work.
lastly, unless it's sweelee, don't have too high hopes on getting discounts. at the most ask for free picks, pack of strings or something trivial, and try ask the salesperson to round the total amount down.
that aside, always speak with respect, because mutual respect does benefit everyone.
my 2 centzzzzz