Read and heed, please. I don't want to have to tell you again.

Two common mistakes I see online and elsewhere:

1. Forget that rule you learned in typing class about putting a double-space after each period. That only pertains to typewriters. When you're writing something that will be set in type - electronic or otherwise - just put one space after a period.
2. When you have close-quote marks at the end of a sentence, they go after the period, not before: As in, "Don't do that." not "For God's sake, never do this".