If the pen might is mightier than the sword, then your credibility runs grave risks every time you sit down to write. Make sure you ruthlessly eliminate these common and easily avoidable writing errors from your work.
1) Misuse of similar-sounding words such as affect and effect, their and they're, to, too and two, you're and your, lead (the metal) and led (to lead, past tense).
2) Using nouns as verbs (or as the saying goes: never verbify a noun). Some particularly painful examples: liaise, medal (as in "he medaled in the Olympics"), or the eye-burningly awful incentivize.
3) Common transposed misspellings, such as from/form, casual/causal. Spellcheckers will miss these, so you'll have to catch them the old-fashioned way: by carefully editing your work.
4) Failing to make sure your nouns and verbs agree, especially in lengthy or complex sentences.
5) Incorrect use of fewer and less: here's the way to remember this rather annoying idiosyncracy of the English language: so-called "mass nouns" get less (I drank less milk than you) while "count-nouns" get fewer (I ate fewer M&Ms than you). Unfortunately, the slogan "a third less calories than the regular beer" is improper English.