Where to start
Don't waste any time. As soon as you spot a fault or experience bad service, bring it to the attention of someone in authority.
Yes, even the 17-year-old sales trainee should realise that a new car with square wheels is unacceptable, but he or she won't be writing the cheque for a refund. Ask to speak to someone who has some clout instead.
Here's the difficult bit: perhaps you've just spent 20,000 on a lemon, or waited all afternoon for a car the garage promised would be ready by lunch - your blood may be boiling, but don't let your frustration spill over.
There may be an explanation for the problem, but even if there isn't, you want help from the person you complain to.
Hopefully, your calm, rational exposition of the facts will do the trick. However, if your first, verbal complaint doesn't have the desired effect, then it's time to regroup and try again.