I generally use
ssh-copy-id to do this.
Usage: /usr/bin/ssh-copy-id [-h|-?|-f|-n] [-i [identity_file]] [-p port] [[-o <ssh -o options>] ...] [user@]hostname
-f: force mode -- copy keys without trying to check if they are already installed
-n: dry run -- no keys are actually copied
-h|-?: print this help
$ ssh-copy-id -i /path/to/id_rsa.pub <user>@<server>
$ ssh-copy-id -i ~/.ssh/id_rsa.pub email@example.com
/usr/local/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/Users/user1/.ssh/id_rsa.pub"
/usr/local/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/local/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
Number of key(s) added: 1
Now try logging into the machine, with: "ssh 'firstname.lastname@example.org'"
and check to make sure that only the key(s) you wanted were added.
NOTE: If you find you need to do this across multiple servers refer to my Q&A titled: How can I propagate my SSH pub key to a list of servers without having to type my password in over and over?.