If your looking for used.. look at nadaguide or kelly blue book.. look at the car.. tell them you DO NOT have to buy it today. Their job is to keep you there as long as possible.
Give them a fake phone number if they are shady and insist.. or transpose two numbers.. they will call you to no end
After you look at it LEAVE decide what exactly you want to pay.. it should not be over retail value in any book. Keep the max number in mind.. start negotiating at a resonable number.. they will counter higher. then give them your number and tell them that is all the car is worth to you. if they want to sell it fine.. if not LEAVE if your interested and close don't be surprised if they call you the next day to see if you will take it at your previous offer.
Also- ask for an OUT THE DOOR price.. depending where you live they can tack on dealer fees etc.. in florida they range around 200 to 600 for their "fee" bunch of BS
Also.. don't give them your keys or any other reason to hold you there..
Remember they are there to get as much of your money as possible for the car.. they are salespeople and that is their job.
your in a good position if you don't need a car "right now"
Have fun... I am about to go shopping as our altima is in "damaged" condition .. wife got to close to another car..