of course people chime in telling you that you cant negotiate... i think this ultimately hurts the lbs in todays market. you are seen as a cheapskate. everyone is a cheapskate. but you have to look at value beyond just the price on the bike, because you pay for other stuff after that too.
i will tell you this from my experience:
if it's last year's model or some wierd size, they might want to just move it out the door and you are almost doing them a favor by buying. but you won't get any real discount. also, dont expect a discount this time of year (spring). late fall you can find deals because bikes are not moving, but that isnt true near christmas. feb/march is a great time to find a deal on last years bikes. i had a shop owner tell me i was doing him a favor by buying a bike at cost from him, just so he can get it out and pay the rent. he had to sell it for less, as it wouldnt sell sitting next to the current year model for the same price.
i have also found that you can get a deal from the LBS by having them throw in some accessories for a discount, because I think they have a larger markup. so if you need shoes or something get it when you get the bike.
i have also had good experiences with having the shop fit you to the bike, just in terms of swapping out the stem and adjusting saddle position. they often do this for no charge and it can be night and day on how well the bike rides. this only holds for a good pro shop. since they have such good service and knowledge, they might not need to discount so much because they get sales anyway.
finally, i have found the shop guy to be happier when i pay cash/check. he said he gets dinged pretty hard by credit card fees (anyone know what %?). so when i paid cash he threw in some spare tubes. i mentioned i get 1% back from the CC company and he quickly offered to give me 1% off if i paid cash! could be some 'tax purposes' in that , too.
i have also bought used bikes on the internet with mixed success, mostly good. even with some of the 'bad' ones, once i replaced the iffy parts i still came out ahead, but it was a pain the butt to sort out.