Seriously though, it's simply a product of:
a) reading a *lot* of what I'd call "good quality literature", and
b) taking the time to proofread+fix my posts before I make them.
The first helps because it means I've spent the last 20-odd years soaking my brain in good grammar (so it comes pretty naturally). The second helps because even the more carefully thought-out sentence can get mangled by fingers mashing the keys too quickly.
I generally re-read a post twice after doing any typing-- first from top to bottom to make sure it flows correctly, and then from bottom to top (one paragraph at a time) to make sure my brain hasn't glazed over an obvious error. If I make any changes after the proofread then I re-check top to bottom to make sure the flow still makes sense.
I guess it's a little obsessive, but it builds/re-enforces good writing habits; plus, given that the written word is the main form of communication in internet land (without any of those helpful things like: body language, facial expression, or tonal variation) I feel it's not a bad thing to try and ensure your intent is expressed as clearly as possible.
I also concur with FBB on the point of reading helping.
I mean, hell, I probably had about the same vocabulary as I do now 1-2 years ago, and I was only in the 5th or 6th grade or something.
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