Bastille's Official Home: http://www.bastille-linux.org/
Interview with Bastille's Jon and Jay: http://slashdot.org/interviews/00/11/08/1616204.shtml
Lance Spitzner's Article “Armoring Linux” (includes a handy list of startup scripts and what they do, which is useful for figuring out which services to disable): http://www.enteract.com/~lspitz/linux.html
The Linux Documentation Project's Official Linux Security HOWTO (less a detailed procedure than a big list of resources and where to obtain and find out more about them): http://www.linuxdoc.org/HOWTO/Security-HOWTO.html
Last and definitely not least, you must keep current with patches and security bulletins. Visiting your distribution's web site frequently is a good way to stay current.