The most common thing to try is a content caching plugin like W3 Total Cache or WP Super Cache. Both will cache entire pages to disk, and will allow the Apache web server to completely bypass PHP processing for many page views. With W3 Total Cache, you can also cache system objects and the results of database queries, as well as using a CDN (Content Distribution Network) or a local memory cache server such as memcached, APC, or XCache. Additionally, it can perform combination and minimization of your CSS and JS, which can be a performance win (by reducing the number of requests a browser must make).
Other strategies include combining images into a single sprite image, and using CSS display only the appropriate portion. This reduces the number of separate images the browser must download.
Other possibilities, which require a lot more work/knowledge/resources: Switch from Apache to Nginx or some other high-performance web server. Switch from mod_php to mod_fastcgi + php-fpm. Implement multiple backends for your web server, with a high-performance caching proxy server in front. Multiple MySQL servers.