Some areas to work on:
- The price of similar spec cars via say Autotrader. Make sure that you've actually done the research though. Pointless trying to compare different spec cars and you'll quickly have the rug pulled from under you by any clued up salesman
- Check the car you're considering buying in detail, ie. it's obvious condition (mileage, paintwork, tyres, oil condition on the dipstick, any knocks, panel gap mis-alignment etc), then also consider when it next needs servicing, how much that will cost etc. That can sometimes open up an area of negotiation
- As already stated, be prepared to walk away and say no
- Lastly, and this is a personal opinion, buy from someone you actually like. I remember reading years ago that it's good to do business with people you LIKE doing business with. If they're offhand and don't give a stuff when selling you a car, imagine what they'll be like if it comes to after sales care and similar. Additionally, my experiences are that the people most likely to shaft you by covering up dodgy histories about their car and also typically the least pleasant people when you first meet them. Finding someone who's actually a NICE person to deal with is NOT impossible.