Your best bet is to have the firm number in your head that you want to pay, and not even use the word "invoice"...then, visit or talk to several dealers and find the hungriest one...tell them that you know about holdbacks (incentives the manufacturers give the dealers to move vehicles in a certain timeframe after delivery, etc), advertising monies, financing kickbacks, etc...their true cost is below "invoice", but there is no real way to compel them to sell it to you for that (after all, they ARE in business to turn a profit, this isn't a hobby). You just have to find the dealer who wants your business bad enough to bend...and don't expect them to do any of this "bending over" on the phone or over email - you have to look serious and capable of doing the deal - go down to the lot and have checkbook in hand, ready to do a deal...face to face may not be the most comfortable for some people, but it's the only way the dealer knows you're not just yanking his chain on the internet of phone, just another cross-shopper wasting his time...
Bottom line is know what you want to pay and don't budge until you achieve it...in today's market, the buyer is king...having said that, don't expect them to sell everything at huge losses either...remember, it's a two way street.