In the following sentence what would be the correct punctuation
One more thing don't tell anyone about our conversation.
Should "One more thing" be separated by comma, dash or colon? Or perhaps nothing at all?
The simplest and the one I would prefer is to treat it as two sentences:
This also gives you the facility to maneuver the second part.
Where the second part is short, and more important, use a colon to direct attention:
Where the whole thing is rather simple and short, just include a comma:
[EDIT] additional info, the em dash.
A sentence like that would not be appropriate in formal writing at all. If you are reporting it as direct speech, a comma after thing would represent more or less accurately the way it might have been delivered.
All three are fine in informal writing. I prefer the first.
"One more thing." is not a complete sentence. Therefore, it should not be followed by a period or a semicolon. Remember that a semicolon connects two stand-alone sentences that make more sense together.
I agree that if the second clause is more important, then you should definitely use a colon. However, the longer a clause is, the more ease can be gained from varying your punctuation. If your sentence is shorter, you can easily get away with using a comma. If your sentence is longer, or contains a list which must be separated with commas, you should use a colon.
The em dash is very useful in this case, since there is often a much more prominent break between the clauses when using this phrase.
In formal writing, "One more thing" as a clause by itself should not be used. Instead, you could write "There is one more thing." or "..., because of one more thing." or something of the like, in which case you can use a period or semicolon.