Couple good ones from grandpa poop:
"Sometimes free is expensive."
"Never get in a big hurry to spend a whole lotta money."
"Something's only 'worth' what someone else is willing to pay for it."
Personally, I'm getting to the point where I really enjoy buying stuff from insurance auctions. While it requires a fair amount of legwork, and is definitely a crap-shoot, I have a pretty easy formula: What it's worth in scrap plus what parts are worth minus cost to get it home minus cost to get it tagged, and if necessary, have a key cut = what I'm willing to spend. This has worked out really well lately. I've scored 3 running cars for less than $300/each in the last year this way.
For the average shmoe who's not interested in doing the auction thing, I'd focus on the basics: Check for leaks, check fluid levels and condition, look for maintenance records, decent tires, make sure expensive maintenance that's due per the mileage has been done (t-belt, water pump, etc.,) if not, subtract it from the price of the car as part of the negotiation. Also, generally speaking, a car that's obviously been loved on the outside (clean, waxed, generally good looking,) is more likely to have been maintained well.
Never negotiate over the phone/via email. Bring as much CASH as you're willing to spend (or a money order if you're shopping in a E36 M3 neighborhood.) and be willing to walk away. Be polite and respectful to the seller. "Would you take 'X' for the car" comes off a lot nicer than "I'll give ya 'X' for this piece of crap." Be leary of buying a car from someone who rubs you the wrong way, or talks the car up to be something more than it is.
Oh, and as a general rule of thumb, I stay the berkeley away from buy-here, pay-here lots.