We always buy new but typically keep our cars for 12 to 16 years. Since my father-in-law works for GM, we always get GM employee pricing. Plus we always wait until there is a GM rebate to purchase a car and since we have a GM mastercard, we usually get a rebate from that as well. For instance, we bought an almost fully loaded Cadillac CTS (didn't buy the navigation system) in 2009. The sticker price was around $45K. With GM employee discount, $3K GM rebate, and $1.5K GM Mastercard rebate, we paid just over $32K plus TTL. Basically what we'd have paid for an almost 2 year old CTS.
Another example was a 1991 Geo Storm GSI we purchase (bought it during Gulf War I when car sales tanked). I drove it for two years and really didn't like it, so I ended up selling it for $500 less than I originally paid for it. Then it was basically the same kind of deal as the 2009 CTS, but in 1993 I bought a Buick Regal GS sedan, then drove it for 16 years.
I guess I look for a car with the most features, is comfortable, nice interior, and nice smooth ride. Gas mileage isn't usually in the equation when I pick a car because we drive so few miles each year. Our fully loaded 2003 Trailblazer (8 years old) has 72K miles, and our 2009 CTS (2 years old) has 9K miles.