Thirty-four years ago in high-school typing class, I learned to hit the space bar twice at the end of each sentence. This was good advice, because my instrument was an Underwood typewriter (not as old as this one, but almost).
Today, writing on a computer with proportionally spaced typefaces, one space is enough; two is too many. For reasons why, read One Space or Two?, by Ken Adams.
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p.s. (11/2/06): See also Ruth Ann Robbins, Painting With Print, 2 J. ALWD 108, 129 (2004) (footnotes omitted):
The practice of using two spaces between sentences and
indented first lines to begin the new paragraph are merely remnants of days
when attorneys had only typewriters at their disposal and were forced to use a
monospaced font. Using a monospaced font requires two spaces between
sentences in order to provide enough visual cueing through width. But
using two spaces with proportionally spaced fonts will create extra gaps,
which will cause the reader to experience a greater fixation pause between
sentences as her eye searches for the next phrase.