To use multiple public/private keys with ssh, instead of having to specify which key you want to use for which host on the command line, you can create a $HOME/.ssh/config as such:

# Default Identity to use
IdentityFile ~/.ssh/id_rsa

# I create a different key-pair for each host
Host heroku.com
IdentityFile ~/.ssh/id_rsa-heroku

Host github.com
IdentityFile ~/.ssh/id_rsa-github