I'll go over basic to intermediate level techniques to use varnish and ESI to accelerate your rack/rails/sinatra app without adding any beef to your datastore or web app. Before you go horizontal, get the most out of what you already use. Even if you're already familiar with the core concepts, you may be surprised at the scope of what you can do to improve app performance.
For the uninitiated:
Varnish is a high performance memory based HTTP cache that can make your end user experience snappier and drastically reduce load on your back end components.