An article at the 13th Parallel website, discusses ways to set up your content, use style sheets, and add behavior controls for different browsers to maximize the usability and accessibility of a site.

The basic steps:

1. Write your content
   
2. Implement the W3C standards
   
3. Define the appearance of the pages
   
4. Apply behaviors
   
5. Test for accessibility

One goal is to have the pages degrade gracefully when someone visits using an older or non-compliant browser. For a good example of this in practice, visit the 13th parallel article in IE5 and in Netscape 4.7.

...it's a useful article, BTW, even if they can't manage to spell 'separate' correctly.