This is not really Mac-specific, but still a good idea. I like two things: sharing and encryption. But you can't have both with wi-fi. Either you share it or you encrypt the OTA transmissions with WPA.
My compromise, which works for anyone in an apartment building is simple. Name your network like this:
Bring beer to [apt number] for Password
It's a nice way to get to know your neighbors, gets you free beer (pro tip: you can substitute other items for beer), and lets you be generous with your wi-fi while giving you some added level of security from the WPA encryption.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090902131809240