One old trick that I've used in a pinch is to create a single pixel transparent GIF. Stick the gif inside the div and set it's width attribute to the minimum width you want for the div. Think of it like propping open the div with a transparent stick.
Doing this allows the div to grow wider but it stops shrinking when the div reaches the ends of the gif.

When used in say your header div, this method has the effect of giving you both a flowing and fixed width layout. Some of the advantages of both worlds.

There are better ways of fixing issues like yours but it can be another tool in your toolbox.

This of course won't work if you have scroll bars turned on in the div.