What I do, Mr Larry.
Before I click Post, I highlight all the text and press Ctrl and C simultaneously to copy it into the temporary memory buffer. (Actually, I Ctrl-C twice, because sometimes it doesn't work the first time.) Then if the worst happens, I open a new reply window and just Ctrl-V to paste all the lost text back in.
Oh, once I didn't press Ctrl properly and all my highlighted text was replaced by a single forlorn-looking lower case "c". Ctrl-Z got back everything I copied over. Of course, the real trick is typing everything without typos. The proof of the post is in the proofreading.