The other answers assume you've written a CGI/modular apache application - I'll assume you've written your own custom application that also listens on port 80, for purposes of administration.
On a Linux box, the simplest method (not involving having to write your own .htaccess ACL system or similar), is to use iptables to prevent anything but local access to your port of choice:
iptables -A INPUT -p tcp --dport 80 -s 127.0.0.1 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
Obviously this is better served to put into an init script that will load on boot, and could potentially be made more robust, but for your current limited purposes as defined, this should do the job. :)