Are you referring to a resale "new" home or a new home as being purchased from a builder? There is a major difference that may require a variation in your approach.
New construction sales agents try to remain commited to their advertised pricing for the obvious reason of not creating a builder devaluation of home previously sold in the community. To decrease the sale price in an established area could have serious implications on the values of other area homes as well as the builder's business.
From time to time, builders will offer sales promotions. These opportunities should be the source of your negotiations. Understanding the models available, pricing variations & history, upgrades, construction, promotions etc. may help to create a window for negotiations.
There is no substitute for not having an experienced RE professional that is familiar with new construction as well as all competitive products similar to your taret property.
When relating to the sales staff it may be helpful to identify yourself as a motivated buyer(have a letter of "pre-approval" available) but an informed buyer that is exploring their options thoroughly prior to making a decision. Builders do not like to lose a serious potential buyer to a competitor and will do what they can to win you over.
If your intention is to identify and purchase a new construction there is no substitute for collecting information and becoming as informed as possible with all of the area's similar products.