ssh -L 3333:foo.com:5432 joe@foo.com

I think this should be changed to

ssh -L 3333:localhost:5432 joe@foo.com

The reason is that this assumes the postgres server on foo.com allows
connections from foo.com, which is not allowed by the default
listen_addresses setting.  Add more detail explaining this.

pointed out by Faheem Mitha

Also change the example port number 3333 to 63333 so no one can complain
that we are stealing a reserved port number.