Rocky (the US prestashop Moderator) have created a nice page for this. You could check how to improve prestashop performance at his website here
The page covered eight topic for prestashop performance.
- Enable GZIP Compression and Browser Caching
- Combine, Compress and Cache CSS and JavaScript
- Enable Smarty Template and File Caching
- Optimise Images and use CSS Sprites
- Serve Static Content from a Cookieless Domain
- Reduce Cookie Size
- Use a Content Delivery Network
- Divide Product Images into Subdirectories