Best way to do it, imho, is right click the 'post reply' link in a thread you wanna post to, and then click 'open in new window' (or similar, depending on your browser).
Then after you've typed your reply in the new window/tab, hit 'post' once and return to the original thread (in the other window/tab) while the cogs are turning on your new post.
Clear ?
/.02
Thanks for that reminder - I used to do that all the time on the hurricane threads and I guess being away as long as I was I totally forgot. Thanks, again!