externalip=myaddresshere.onion #tells the client to advertise my hidden service address in addr messages
discover=1 #tells the client to find out what its IP address is, despite externalip
listen=1 #tells the client to listen for incoming connections
tor=127.0.0.1:9050 #tells the client to use localhost:9050 as a proxy to connect to hidden service addresses
For more, take a look at https://github.com/bitcoin/bitcoin/blob/v0.8.5/doc/Tor.txt -- that lists the options as they exist in release 0.8.5. The link dserrano5 is mostly correct, but it's correct as of current git head.