How do I permanently add an identity for SSH?

Generate your key like normal: ssh-keygen, then place that key to the remote server with ssh-copy-id, which will sync it to the remote server's accepted keys.

ssh-keygen
ssh-copy-id user@host

It will prompt for your password then perform all the steps necessary to link your .pub key with the remote SSH server.

By default it will copy all your .pub keys to the remote server. If you just created your key with ssh-keygen then this isn't a problem (because you only have one!). However, if you have multiple keys you can copy just a specific key with the -i flag.

ssh-copy-id -i ~.ssh/key_name.pub user@host

Replacing key_name.pub with the name of the key.

put this in your ~/.bashrc

eval $(ssh-agent)
ssh-add ~/.ssh/where_ever_privake_key_is 

You can generate a ssh key with the command:

ssh-keygen

Then you can copy your key to the server with:

ssh serveruser@servername "echo `cat ~/.ssh/id_dsa.pub` >> ~/.ssh/authorized_keys" 

Now you can automatically log in your webserver

If your key is password-less and named as one of the files ssh will try to look for when identifying (~/.ssh/id_dsa or ~/.ssh/id_rsa), you shouldn't have to add it to your agent.

BUT. If there's the slightest possibility of those files being stolen, you would have just allowed anyone to access the servers on which you are using this identity. In short, pwned.

IMHO, password-less private keys are a bad practice, and should be used only on environments where ~/.ssh/authorized_keys is very restrictive.