I’m all about single spacing after sentences. I realize double spaces were standard during typewriter days, but spacing twice after punctuation marks at the end of sentences is just not necessary anymore—unless you’re writing or editing an APA-Style paper, in which case you should definitely hit the space bar two times. Why? Because the extra room aids readers of draft manuscripts.
Still, you should insert only one space after commas, colons, and semicolons; periods that separate parts of reference citations; and periods used with initials in personal names. Oh, and don’t use internal spaces at all with abbreviations like a.m., p.m., and U.S. And when you end a sentence with an abbreviation, like I just did with U.S., there is no need for another period.