ssh-add: How to add ssh keys permanently

Question

I'm using Ubuntu 12.04 and every time I reboot the machine, the key identities saved by ssh-add are gone.

~$ ssh-add -l
The agent has no identities.

I have already added "IdentityFile" in my .ssh/config file like the post below instructed, but it's not working.

Add private key permanently with ssh-add on Ubuntu

My .ssh/config file looks like the following:

Host github-user-1
    HostName github.com
    IdentityFile ~/.ssh/id_rsa.git.user-1
    User git
    IdentitiesOnly yes

Host github-user-2
    HostName github.com
    IdentityFile ~/.ssh/id_rsa.git.user-2
    User git
    IdentitiesOnly yes