If you generated your ssh key pair, did everything per instructions and still can't log-in using the keys, you may be a victim of the most common ssh key problem: incorrect permissions. Permissions both on the server and the client machine (if your client is Linux/Unix or Mac OS-X) must be set properly, which is:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/authorized_keys2

Also, don't forget that while ssh-keygen generates id_rsa.pub file, when uploaded to the server, that file must be renamed to: authorized_keys2