How to negotiate when buying a new car

I have bought 2 new cars over the past 7 years.
Paid a whole lot of extra money the first time, learned and got a better deal the second time. In both cases, I already loan approved from my bank.

Car #1 Toyota Corolla, base model
Knew nothing, went to the dealer and got the car when I got my first job.
Paid extra for the car, random insurance that they added, other stuff. Total rip off.
Paid at least $5,000 more than I needed to.

Car #2 Honda Accord
Was not going to make the mistake again on going to the dealership to directly buy the car.

First decided exactly what I wanted. Went to a dealership with no intention of buying the car on the same day, but needed to test drive to zero in on the model. Decided on the Honda Accord EX 4 door 4 cylinder silver color car. Got the quote and told them will decide in a couple of weeks.

Went home, and created a spam only email address. With the exact model, color, etc of the car emailed all dealership in the 25 mile radius to get the quote. DO NOT GIVE YOUR PHONE NUMBER TO THEM, ELSE THEY KEEP CALLING.

Got quote from most of them. Took that lowest quote and forwarded it to the others if they could match. Continued this till no one would go any lower.

In some emails they will lure you with come to the dealership and so you can negotiate. Do not waste your time going to those dealership. Get a written number from them in the email, and only then go, and be prepared to walk out if they don't agree on the number. Also do not give any SSN, etc till the deal is finalized.

Another trick, if the dealership closes at 6 pm, go there at 5 pm. They are in a rush to leave and won't sit around negotiating much, and will also do the paperwork quickly.

Toyota Corolla: Paid $22,500 for the base model out of the door. Ideal price should have been ~17.5-18k
Accord: The dealership where I did a test drive, quoted me $28,000 out of the door. Negotiated for the same model over the internet, and got it out of the door for $23,700.

Told this trick to a few friends, and they all saved a ton of money. Hope this helps.