Upstart. The beauty of it is that it's much simpler to setup than the old init.d way and also easy to use. If you want to run Thin and have the processed managed by Upstart, here's the step by step.
This assumes you have a Rack based app ready to go.
1. Create an Upstart config file
Change the APP_HOME environment variable to point to your Rack app.
Paste it into a file at /etc/init/thin.conf .
2. Start it
sudo start thin