Why are off by one errors so common and what can we do to prevent them?

Question

It seems that off-by-one errors are one of the most (if not the most) common programming errors (see https://softwareengineering.stackexchange.com/questions/109/what-are-common-mistakes-in-coding, and conventional wisdom).

What is the reason these are so common, is it something to do with how the human brain works?
What can we do to prevent falling prey to the off by one errors?