Use a tabbed browser (IE7, Firefox, Opera, Safari).

Middle click on the post button, this will open a new tab with the post page. Write you message, post it and close the tab. The original page with all your new tags will remain unchanged.

That should say, middle click on the reply to this link not post button.
Thanks, I've used that method in the past and it looks like I will have to use it again.

It's not just a problem with posting, BTW. It appears that if you do anything that either goes to a different part of the page (i.e. clicking on the "Parent" link) or brings up a new page (i.e. "Reply to This," "Post a Coment," etc.), then try to skip back to the page you were reading using the back button on your browser (usually Alt-Left Arrow, but you can also skip back multiple pages via the drop-down back menu), then you lose all your "[new]" tags. I think somewhere along the line the page is being forced to reload, which blows away the old "[new]" tags and brings in the next set.

This is a recent change in behavior, and I can't figure out whether it's in my browser (Mozilla Firefox 2.0) or a recent change in the Oil Drum web site.

I think it's your browser, because "back" still works for me (in Firefox 2 and 1.5).  

You might trying emptying your cache and/or increasing its size.