I can only think of one...
Choose a list of ~100 ip address that are constantly "connectable" found by tracking this graph every week or two:
http://maps.google.com/maps?q=http://c1958612.r12.cf0.rackcdn.com/bitcoin.kml (not actually that graph, because it is a backup,
apparently the live version by MagicalTux is down right now)
and add them to your firewall? To only allow those IPs to connect to port 8332 or whatever port you were running bitcoind on?
Another option would possibly be changing (to say port 52395 or something) and hiding your bitcoind port, creating the same firewall mentioned above, and marking it "invisible" to any other "port scanners" or what not? That way nobody would even know it existed or that the server was online (unless they were one of the 100 people you automatically connected to).
Ideas? Thoughts?
