If you ever have developed a website which has tasted a bit of success and has reached to serious volume of traffic, welcome to the Big boys club and new headaches!
More traffic means high server load which forces you to take number of steps, some of the most important are listed here (in no particular order):
- Make fewer HTTP requests
- Use a Content Delievery Network
- Add an Expires header
- Reduce DNS lookups
- Enable Gzip Compression
- Enable MySQL Query Cache
- Cache website server side processing such as cache_lite for PHP
Out of the above mentioned steps, caching your database queries and server-side processing will make the most difference to your server performance. Unless you cache your content processing, your webserver and DB server is not going to take a sigh of relief, and niether will you as webmaster.