Don't use "you" when you mean "thou"
This is a common mistake actually, so common that it's not considered a mistake any more.
The fact is that "thou" is the second person singular, and "you" is the second person plural.
Here's an example:
"Art thou going to the theatre with me?" - That is only one person, it can't be mistaken for:
"Are you (lot) going to the theatre with me?" - In modern English, this could be either singular or plural, (hence the "lot" which I added to make it plural for this example.)
Yeah, thou arst a poncy bastard if thou actually speak like this, but at least take note.
No offence to god though..
You know what's funny? I just realised I made the mistake I noded about in my title. Har Har.