How many spaces do I use after punctuation?

When I do proofreading, I notice a lot of confusion about whether to put two spaces after a period (full stop, if you're a British English speaker) or other punctuation used to end a sentence or just one space.  This used to matter more than it does now.  The difference is actually one of British vs. American style.  Historically, British English required two spaces after a period, question mark, exclamation mark, or colon, and one space after all other punctuation, but American English preferred two spaces after all of those but one after other punctuation.  More recently, the American preference is to use only one space after all punctuation.  I know that some British style writing is trending toward simplification such that only one space will be used after all punctuation (and they're even paring down on using commas, just as a side note!).  However, unless you're writing a manuscript where there are strict publishing guidelines, you can always use your personal preference, within reason.  I personally still use two spaces after all punctuation that ends sentences (? ! and .) and one after all others ( : ; and ,).  I think that's how I was taught when learning to type on a computer, so that's what I went with.

!;,.:?

And no, I'm not swearing!