Looking for advice setting up a new Mac Mini with Lion Server
Although written for 10.5, this guide by Daniel Cuthbert (PDF) is fairly comprehensive and largely still applies to Lion.

Closing off screen sharing and tunneling it (if you need it at all) through SSH is probably a good idea.

Standard rules apply:


Only open absolutely necessary ports to the outside world
Keep your software up to date
Disable any unused services &amp; Apache modules
Disable any listeners you don't need, rebind those you do need to localhost only if they're not needed outside (lsof -i is your friend)
Check that your firewall is really working as expected (nmap from an outside host)


Mac specific things:


Make sure the console is password protected, locks if you're using screensharing and doesn't auto-login on startup
You may want to enable verbose boot messages by default, it'll make debugging easier in case things go bad: sudo nvram boot-args="-v"