Negotiation is less about confidence than preparation. Almost everything that determines the outcome happens before the conversation.
Negotiate one number only
Ask for a drive away price, in writing, including all on road costs. Comparing quotes structured differently is where most buyers lose track of what they are actually paying.
Keep the trade-in out of the conversation until the purchase price is settled. Two separate numbers are far harder to obscure than one.
Know the market before you ask
Advertised price tells you very little. What matters is what the model is currently transacting for, whether there is factory support this quarter, and how much stock is sitting nationally.
Approaching several dealers in different regions with the same clear brief will reveal that spread quickly, and it changes the tone of every subsequent conversation.
The lines that are actually negotiable
Dealer delivery and administration charges vary widely between sites and are frequently movable. Accessories carry more margin than the vehicle. Protection packages, tinting and extended warranties are almost always negotiable and often available cheaper elsewhere.
Servicing packs are worth assessing on arithmetic rather than instinct, since some genuinely save money and others simply prepay it.
Timing, and the mistake to avoid
End of month, quarter and plate year all influence dealer willingness, as does the runout of a superseded model. If your timing is flexible, it is worth real money.
The most expensive mistake is emotional commitment. Once a specific car in a specific colour is the only acceptable outcome, the negotiation is effectively over.