For multiple services like:
Host github.com
HostName github.com
PreferredAuthentications publickey
IdentityFile ~/.ssh/github_rsa
Host bitbucket.org
HostName bitbucket.org
PreferredAuthentications publickey
IdentityFile ~/.ssh/bitbucket_rsa
# generate ssh keys
ssh-keygen
# add passphrase, etc
# add passphrase and repeat for n keys
ssh-keygen -t rsa -f ~/.ssh/service_rsa
# change permissions inside .ssh/ folder
chmod 600 *
# add keys to services
# if there is only one account per service no config file is required, otherwise a config file
# inside .ssh keys
# on the machine add ,by add for each key.
ssh-add ~/.ssh/service_rsa
# list current added keys on local machine
ssh-add -L