You should be able to find the invoice price off the internet, and also find the holdback, if it exists. Imports didn't use to have holdback, but some do now -- don't know about Nissan.
I'd start by offering about 10% less than invoice-holdback, or 15% less than invoice alone.
If you have good credit (anything north of a 650 score), they will put you in a vehicle for right about their cost -- if they think you are going to finance with them. So don't mention a credit union, even if you want to use it. In fact, use that as a negotiating ploy -- get them competing against the credit union.
I've always bought cars late in the month, so managers who are trying to pull specific numbers will approve what they might not before then. Also, negotiate everything: any aftermarket accessories, the extended warranty, the financing, etc.
The final thing to remember is that you will tire of the process long before they will. If you have the stamina, you can get a car for almost nothing (over cost).