Always use a reputable dealer.
Do your homework, if the dealer won't give you "figures" ie. the on the road price for a new car, go somewhere else. there are plenty of garages who will. If you can go to a dealership with one or 2 quotes that you have obtained from a garage or the internet, most dealers will bend over backwards to try to beat the price quoted.
The cash price is not always the best price, some manufacturers will offer incentives for you to take their finance, and as you can pay off the balance at any time, you may get a better deal if you take up their offer.
You are always going to loose money when buying a new car, (even before it leaves the showroom) as soon as the vehicle is taxed you pay 20% VAT, and have further depreciation as it is no longer classed as a new vehicle.
Why not look for an ex demonstrator,or pre registered vehicle,( many manufacturers have big incentives for dealers to register vehicles, especially as it gets closer to the end of each quarter, these are known as pre registered). mostly they only have delivery milage. also dealers have access to manufacturer vehicles ie Ford direct, Vauxhall management etc. ask the dealer for details.
If you buy a car privately, you have very few comebacks if there are any problems, the phrase buyer beware springs readily to mind.
If you buy used from a garage, always insist upon a 12 month warranty, extend it for as long as you can afford, warranties are there to protect you, you get what you pay for, ask friends/family who they bought their cars off, if they have had agood experience you might find that you do as well.
Alway ask for proof of a Hire Purchase check. All reputable dealers will do this as a matter of course when buying a vehicle.
As for trying to twist the arms off a dealer, its the one thing that gets the dealers back up. You buy a car once may be every 3 or 4 years, these guys are selling cars every day, if you are not careful, you get what you think is a good deal and the dealer is the one who has the last laugh. be reasonable, ask for a discount, if the dealer thinks that you are buying there and then you will get his best deal.if you are not happy walk away.....