Whenever I have bought a new car I always get the latest Car Price guide from the newsagent. Dealers use their "secret" Glasses Guide but the numbers are pretty much the same. I take the guide with me and make sure the salesperson sees it - they always comment something like "I see you've done your research then". I bought my wife a new (2nd hand) car in May. I used "Parkers Car Guide". If you know the reg. and the mileage of the car you are interested in, you can send a text to Parkers with this info. and comes back within seconds giving you confirmation of the car's details and the different prices, dealer, part ex etc. I showed the salesman this on my phone so he new what price I was looking for. Without any haggling he offered me a full tank of petrol (worth about £60 at the moment) 12 months warranty, 12 months MOT, and Paint/Leather protection which meant he got his price and I got a good deal - a win/win situation.
The text costs £3 from memory but well worth it when you are spending thousands.