<p>Generally, what's the best way to tackle a layout bug in IE6? What are the most common bugs or problems that one should look for when trying to figure out why your page suddenly looks like a monkey coded it?</p>

The box model is usually the culprit.  Basically what this means is that any div you are trying to position and use unsupported CSS with will cause this problem.
You may find it happens if you are using min-{width,height} or max-{width,height}.
this provides a great reference for checking compadibility with different versions.
http://www.aptana.com/reference/html/api/CSS.index.html