My (now) wife bought a new car late last year. No finance. I let her haggle. Well, she insisted I watch. :-)

* Be thankful *you* are doing the shopping, not your wife.
* Get a quote in writing, including all the bits-n-pieces they tack on. It is horribly easy to forget the extras, because they add them on right at the end. We got stung nearly $1000 by this.
* You have to be willing to walk away right up until they have your money. (This is very hard! We couldn't do it right near the end, though we'd bargained hard all the way through.)
* They will try to sell you things like paint-protection, burglar alarms, rust-proofing, yadda yadda. After you've committed to the sale. I felt cheated when I bought my 1yro Lancer because they tried this and told them so, but I couldn't talk my wife out of it when she bought her new X-Trail.

Wade.