I know you've all seen variations of the following list, but really it's just too funny yet full of good tips :D I found this version at Grammar Humor.
Don't use a big word where a diminutive one will suffice.
Don't use no double negatives.
Don't never use no triple negatives.
No sentence fragments
Corollary: Complete sentences: important.
Stamp out and eliminate redundancy.
Avoid cliches like the plague.
All generalizations are bad.
Take care that your verb and subject is in agreement.
A preposition is a bad thing to end a sentence with.
Avoid those run-on sentences that just go on, and on, and on, they never stop, they just keep rambling, and you really wish the person would just shut up, but no, they just keep going, they're worse than the Energizer Bunny, they babble incessantly, and these sentences, they just never stop, they go on forever...if you get my drift...
Never go off on tangents, which are lines that intersect a curve at only one point and were discovered by Euclid, who lived in the sixth century, which was an era dominated by the Goths, who lived in what we now know as Poland...
Don't obfuscate your theses with extraneous verbiage.
Never use that totally cool, radically groovy out-of-date slang.
Avoid tumbling off the cliff of triteness into the black abyss of overused metaphors.
Keep your ear to the grindstone, your nose to the ground, take the bull by the horns of a dilemma, and stop mixing your metaphors.
Avoid those abysmally horrible, outrageously repellent exaggerations.
Avoid any awful anachronistic aggravating antediluvian alliterations.
This sentence no verb.