What is the origin of mind your P's and Q's?

What is the origin of mind your P's and Q's? Various theories have been put forward. Some say that it was a warning to printers apprentices when sorting type, the letters p and q being almost identical in form, others that it was used in inns, the amount owing being chalked on a board in order that customers should not order more than they could pay for when settling day arrived.
(A recent snippet from my main blog)

Riches and poverty

Poverty does not guarantee blessing and riches do not preclude it.

You'll stay like that

Like many a child I enjoyed doing this when I was young. I would often be told that I would stay like it. Some say it will happen if the wind changes. I don't think it's true.

Inconsistency

Being inconsistent is okay, as long as you don't do it all the time.