I had an issue with CSS today and it was difficult to figure out what exactly what was going wrong. A great way to avoid this is to reset everything before you start.
A good way to do this is to use a preset reset such as the Yahoo UI reset or the Eric Meyer reset.
While these resets are very comprehensive often it feels like you reset everything, only to then redefine a lot of properties on the elements. Eric Meyer recommends that you should not just take his reset stylesheet and drop it in your projects if there is a more effective way of using it. Tweak it. Build upon it. Customise, and make it your own.
Perhaps it could be something as simple as removing padding from the elements:
html, body, div, h1, h2, h3, h4, h5, h6, ul, ol, dl, li, dt, dd, p, blockquote, pre, form, fieldset, table, th, td { margin: 0; padding: 0; }