Many of the perks a dealer offers are really expensive, including rust-proofing, paint sealant and anti-theft devices. Shop around before you even approach the lot to find out what local car shops are charging for these services, and then take the quotes you get with you to either get a deal from the dealer or skip those add-ons totally.
When in the finance office, read your final contract thoroughly. Many dealerships will try to rush you through this step, and they may change important information or financing terms as a result. Make sure the terms are exactly what you agreed to. If they are not, ask them to make changes before you sign anything.