On PHPMaster.com today there's a new tutorial that can help you increase the performance of your application relatively painlessly with the help of the Varnish proxy tool. The article helps you get started.

Varnish is a reverse proxy server; it sits in front of your web server and serves content from your server and no one else’s. Reverse proxy servers are tightly coupled to the web server and can act on messages received from it. [...] Simply put, Varnish does one thing: serve web content super fast.

You'll need command line access to the machine (as well as permissions to install the software) but getting it installed is a simple "apt-get" away. There's a bit of configuration to set up to get it up and working, but it's only a few lines...and examples are included in the tutorial. They get into some of the more advanced configuration options too, like the time-to-live and changing the port it listens on. Varnish isn't just for PHP applications either - it can be used effectively for any kind of web application as it's just a proxy layer that sits on top and waits for requests.