I find myself needing to do this on a regular basis, so here’s a handy snippet for adding your public SSH key to a server’s authorized_keys file, assuming your public key is at “~/.ssh/id_rsa.pub” (the default).

ssh user@host "echo '`cat ~/.ssh/id_rsa.pub`' >> ~/.ssh/authorized_keys"

…or pop this in your ~/.profile file:

function push-key {
  ssh $1 "echo '`cat ~/.ssh/id_rsa.pub`' >> ~/.ssh/authorized_keys"
}

and run the following when you need to push your key to a server:

push-key user@host

Edit: Christopher mentions in the comments that you could use ssh-copy-id which is great on systems that support it, however it’s not available by default in OSX.