Learn the art of flattery and mental appeasement. When selling, all you need to do is boost their confidence to outrageous levels, get them in a high, and then reassure their mind that whatever product you are selling is necessary in order for their little mind to be at ease.
All people want is to feel good about themselves and they also need reassurance. You can also bring in a third party, unknown to your mark, to flatter, calm and reassure. Nothing beats solid confirmation of quality and experience from another source other than yourself.
Also, create and further their desire for whatever product you are selling. If they feel they simply cannot live without your product, then they will be BEGGING you to sell it to them, even if you raise the price.
I could go on for years...