#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.