If buying a new car tell them you want to split holdback. That's usually $100-$300 below invoice and I've never met a dealer that wouldn't do it.
Always buy on the last weekend of the month.
Get an appraisal on your car after you negotiated price on the new car. Never take the first offer as they are usually holding back there also.
You get your car appraised at a $10k number from the used car manager meanwhile the sales guy comes to you and says they'll give you $8,000.
If you accept the sales guy will make a commission on the $2,000 he withheld from you while explaining how great of a deal you are getting because of tax credits.