#1 Don't use generic classes and IDs in your theme
This seems counterintuitive at first. Naturally you want to name your toolbar "toolbar" and your content "content", but this will cause nothing but heartache down the road.
The trouble is Drupal and other 3rd party systems already have styles defined for these generic classes and IDs. This will make your styling in Drupal more difficult, and if you use a third party shopping cart it will likely have styles defined for these very same CSS selectors.
One of the most time consuming and critical tasks in Drupal development is styling the output. Drupal modules generally come with a default style sheet which can be both a blessing and a curse.
In most cases you'll just override the default style by providing more specific definitions in your site's CSS. This works fine if you only want to make minor changes but what if you wanted a totally different look?