What are some good tips when buying a new or used car?

Answers from experience preferably

Remember beauty is skin deep!
I start with manufactures reliability records. These are found in Consumers Reports Annual Buyers Guide. They are in most public libraries and most school libraries.
They also contain a list of cars that are desired and ones to avoid.
Then I evaluate what transportation needs I have.
Armed with this I start looking.
Using the used car price book, I find what is a reasonable price to pay for what I want. Then when I have identified those that I want and can afford I start looking.
When I find a car that is on the list of desirable used cars that meets my needs, then I check it out. I open the hood and look in the trunk and check out the interior. If something does not feel right I move on. It could be just a feeling (either about the car or the salesperson). Then again it could be the smell of fresh paint, an engine that is either too clean or one that is too dirty.
I very seldom take them for a test drive, because I trust my gut feelings more then anything else.
Pay attention to the seller, if there is something that just does not sit right, it most likely carries over to what they are trying to sell.
I don't care if you are talking about a new car salesperson or a used car sales person, they are all snake oil salesman. If you are not care-full you will get bit.

when i know the model and make i want, i spend a lot of time looking on ebay, the newspaper and go to the Kelley blue book on the internet to get private party or trade in values so you know what the dealer paid for it (they usually pay less than trade in value). also check the auto trader that is online to see what the going cost of the car you want. also those no haggle car dealerships usually have reasonable prices, you can see what they are charging and use that as idea of what the car is really worth but i would even try to beat that price. i even print out my homework and bring it with me to show them i could get that car for a better deal at this other place. it saved me some time cuz they realized i wasnt gonna pay sticker.
its hard to not buy a car when you already spent an hour going back and forth with the salesman and then the manager. every time i decided to wait on it, they've called me the next day to tell me they can do what i had offered. if you have a trade-in, they usually try to make you think its worthless. so make sure you've done your homework on what your trade is worth. if you have a car that is economic on gas, you may be better off selling it privately.
good luck.