Simple!
1. In the client type "ssh-keygen -t rsa". This generates a couple of files, you're interested in /home/username1/.ssh/id_rsa.pub.
2. Append the .pub file to the remote file /home/username2/.ssh/authorized_keys
3. That's it!
(username1 is the user name in the client, and username2 is the user name on the server)
Check for more detailed instructions:
http://www.linuxhorizon.ro/ssh-wo-passw.html
Cheers!
Pedro.