I've been buying and selling vehicles for 34 years. Yea, I'm that old
. There is no magic answer. The first responder is right on the money .. but only if you want to keep most of yours. Buy at around 3 years old. Depreciation and a good deal will take you down to 40-50% of new.
The Costco program and others like it aren't necessarily the best deal. In 2002 I got their price and then beat it by $1000 on a Mitsubishi.
I've never used Uncle Erik's legal pad method, but that certainly has merit. Especially if you bring a good friend or significant other along with the sole purpose of writing down what THEY hear. Also have to be willing to walk away from a deal.
I've found that new or used, knowledge is your best defense against getting taken. The ability to assimilate new information and being fast on your feet and with your intellect is your second best asset. You have to be able to determine what the market bottom line is and then determine if the person you're talking to can get there for you.
Within the past 6 months I've purchased a new travel trailer and a used truck. For the trailer I worked with a dealership I knew but had not done business with before. I used knowledge, innuendo, fact, competion and fear to get the price I wanted. The sales person, sales manager and owner all acknowledged they would had never and were not likely to let another trailer go for such a slim margin. The truck I purchased I paid a fair price for. If I had been completely healthy (had broken ribs) I would have easily saved another $1000 using these same tactics. Sorry, but success is hard won through experience, not in books or forums.
The two best pieces of advice I can offer are:
1. Always be willing to walk away from a deal. You can never want something so bad that you have to have it
2. Don't be afraid to make a crazy offer. If you set it up right they just might bite.
Deal making is NOT a popularity contest. There are always winners and losers. I won on the trailer deal and decided I could afford to lose on the truck deal. In both cases I had assessed the market and knew exactly where I needed to be. In both cases I made the choice, no one forced their will on me. You need to assume control of the sale, no matter how you succeed in doing so. What works for me is not likely to work for you. But I can sure learn from what works for you!