How to design fast web sites?

The best reference for the design of fast web sites is, IMHO, Steve Souders‘s book on front end optimization. He recommends the following best practices:

• Rule 1 – Make Fewer HTTP Requests
• Rule 2 – Use a Content Delivery Network
• Rule 3 – Add an Expires Header
• Rule 4 – Gzip Components
• Rule 5 – Put Stylesheets at the Top
• Rule 6 – Put Scripts at the Bottom
• Rule 7 – Avoid CSS Expressions
• Rule 8 – Make JavaScript and CSS External
• Rule 9 – Reduce DNS Lookups
• Rule 10 – Minify JavaScript
• Rule 11 – Avoid Redirects
• Rule 12 – Remove Duplicate Scripts
• Rule 13 – Configure ETags
• Rule 14 – Make AJAX Cacheable

I strongly recommend his books, which you can easily find using this link on amazon.

You can easily check the enforcement of these best practices using the firebug extension for firefox, or Chrome developer tools.