Instead of making your main column first you make your sidebar first. It gets it's width value in your style sheet and floats right, so it follows those instructions. Your main section gets full width and floats left. So it uses up as much space as it can beginning with the upper left corner. If it passes the end of the sidebar it'll keep taking up full width, only now it's wider than it was.

That, in a nutshell, is it.

Huh ... for some reason, the divs stack now.

AFAIK london was the first skin to do what you're after. Check out the _main and the style sheet for it at http://skins.b2evolution.net/legacy/0.9/london.zip and see how it did it's thing.

Well, london didn't really give me much, unfortunately. BUT, after deleting everything in the bpost css style and ultimately just adding a margin style, it worked. I tried a process of elimination no one thing seemed to be the problem. Anyway, thanks for your help.