If computers are so smart, why can't they keep us from looking dumb? Nothing ruins the perception of your scintillating intelligence faster than an obvious spelling or grammar goof.
The Elements of Style, by William Strunk Jr. and E.B. White
Step1
Use your word processor's spelling and grammar checking features, but don't rely on it completely because it will only get you part of the way there. Microsoft Word, for instance, wouldn't be able to spot the spelling mistake or the grammar and punctuation errors in the previous sentence.
Step2
Proofread everything of importance at least once on paper rather than on the computer screen.
Step3
Show important documents to someone whose expertise you trust. If they find a mistake, ask them to explain exactly what you did wrong.
Step4
If you've never read The Elements of Style, by William Strunk Jr. and E.B. White, pick up a copy. It's only about 100 pages and you can get it for less than $10 ($5 used).