Don't park where you shouldn't or stay longer than you should. This loophole works every time.
What were the circumstances and why do you think you shouldn't have to pay?
Was it a private parking firm (eg on a supermarker car park), a coucil car park or double yelow line? Were there signs displayed? Were the markings valid ? Were teh bays of the right size? There are lots of sites on the internet that will offer advice.