Writing with skill and accuracy is a special talent, Tony, just like perfecting a good CW fist is. It takes patience and a dictionary at hand to get it right, just like it takes patience and good character separation to be a true brass-pounder. Speed is not nearly as important as accuracy. Just ask any QRP'er...
To proof-read what you've written is like double checking your transmit frequency before keying up. No one will hear you if you're 5 kc off and not many folks will bother to finish reading your post if it's full of punctuation and spelling errors.
Writing with clarity can be done with few words. Don't confuse your sentences with too much glitz and glamour. Get to the point. It's easy to "over-modulate" a paragraph and bore your readers with embellishment, just like a speech processor can ruin your SSB signal.
You don't have to be a syndicated columnist or a poet to write a decent post. You just have to be willing to ask old man Webster for a helping hand if you're not sure of the spelling. Even after 20 years of ham radio, I still refer to my allocation chart to make sure that I'm within the legal boundary of my license class. My Webster's is ragged from 20 years of double-checking.
Above all else, write courteously. Avoid contempt and avoid conflict with your words. Put a hold on your impulse to lash out, just as you hold your tongue to let the maritime mobile make a call ahead of you. No one likes to hear a foul mouth on the band or read one on a public forum. Offer assistance, not aggravation.
Do you ever notice the smile on someone's face when you hold the door for them? Do you notice their frown when you let it slam on them in your hurry to get where you want to go? If you don't notice a reaction to your everyday gestures, how can you expect a reader to notice your everyday comment on a message board?
Take care to make the right impression, on the bands or on the forum. Honey will always be more attractive than vinegar.
73 and good DX,