The is no real good way, unless you block all incoming traffic. (ICMP,TCP,UDP).
Obviously if this is a server, that may not be feasible. You could attempt to block all ports except the one service port you need open, but that service may provide enough information to "fingerprint" your machine. In which case you could attempt to customize the service (say Apache) to report differently.
Check out how NMap does it's OS "fingerprinting" here: http://nmap.org/book/osdetect-methods.html#id307894
