I usually just leave the first 10 or 12 characters and cut/paste the rest. If I'm replying to someone, this cuts it off in the middle of the opening [QUOTE] tag, which looks obviously unfinished. If not, I'll deliberately cut off in the middle of a word, in order to make it obviously unfinished.

(This is in case someone opens the page in that first second or two before I can click Edit / Paste / Save. I'd rather it be something that obviously looks like it needs to be refreshed, and this is simpler than writing something else out in the meantime and then overwriting it, like the "Incoming edit" suggestion.)

Sometimes, though, if my first paragraph can stand alone as a summary of my post and the rest is further elaboration on it, I'll post the full first paragraph, then paste in the remainder. If someone sees it in the meantime, oh well, they get the main point anyway.