Put everything you want in a list (including warranty, slim premium plates and full tank of fuel). Go to the dealer, check the car out, tell him it is a nice car and you are in the market to buy a car today (i.e. ready to sign the dotted line today). Ask for a test drive. Tell him your price (which is say 22k if your limit is 25k). Then he will say too cheap and list out the RRP for each accessory. Then you ask him what is his price. Then he will try to push it back to you and not give a number other than RRP. Then after more empty talk, he should ask you what is the highest price you can offer. Then you say $23500 (for example). If he likes it, he will go to the "back room to talk to the manager".
Once you get it, he will ask for a deposit to secure the car. Tell him $50. Then if you decide to forfeit because of personal reasons, you're only out of pocket by $50. Heck, go $100 if you have to. But don't go too much, you don't want to lose a crazy amount. Some ppl put 5% of the car price, which I think is stupid. $50 is all you need. It is just a "gesture" that you are commited to the sale.
Remember, always be ready to walk out if you don't get your price. If your max is 25k, and he offers 25.5k and no lower, you walk.