Perhaps you really mean "Don't fill out credit applications at multiple dealerships?"
Pay cash. It drives them crazy.
All this consumer credit behavior is people agreeing to be sharecroppers. They work while the CC company gets a share of the harvest.
Better still, save your money until you have enough to buy the car for cash.
Negotiate like mad, set a price, get them to write it down on a sales contract.
Then, when they come to the "ok, so how will be pay for this?" point, whip out your American Express card, and watch the color drain from their face.
Argue - strenuously - that they MUST accept Amex as payment for anything they sell, per the terms of their Merchant agreement. When they deny this is so, pull a copy of the Amex Merchant agreement out of your backpack, with the pertinent section highlighted.
Go back to the service area, and have the cashier run the transaction for $32,413.43, have it go through, sign the receipt, and then enjoy the 32,413 Amex Member Rewards Points to take your wife on a free flight to Disney.
Well, at least that's what I did.