Bake oil changes into the loan for the next X years. Move my first payment out a few months in order to accumulate additional interest on the full value of the loan. Add overpriced warranties without telling me. And the best one was simply upping the price and trying to have me not notice by focusing on the payment. Luckily I had a business calculator with me and could do NPV as fast as he could.

I don't know what the trick was for the GP post, but when I last bought one, they tried to sell me one of the car's standard features.