"I recommend to you to take care of the minutes; for hours will take care of themselves."--Lord Chesterfield
Okay, you've proofread. Now do it again. Proofreading is the last thing you do but whose result is the first thing people see. Always use--but never solely rely on--your spell checker, and even your grammar checker.
DVICE. When proofreading, be aware of American vs. English spellings. And when using your spellchecker, remember that it is only a strategy to give you choices, which are not always the same as answers. Answers come from you. The following often-seen example illustrates the point nicely (author unknown):
I have a spell checker,
it came with my P.C.
it plainly marks four my revue
mistakes I cannot sea.
I've run this poem threw it,
and I'm sure your pleased to no,
its letter perfect in it's weigh,
my checker tolled me sew.