Set up forwarding so you can connect via a bastion host.
For example to set up ssh:
ssh -L 9022:distantip:22 bastionlogin@bastionbox ssh -p 9022 distantlogin@localhost
Or to set up psql:
ssh -L 15432:distantip:5432 bastionlogin@bastionbox psql -h localhost:15432
To load an ssh key into your current bash session:
ssh-add ~/.ssh/id_rsa
To run just one command with a given ssh key:
ssh-agent bash -c 'ssh-add ~/.ssh/id_rsa; git clone git@github.com:foo/bar'
On my Xubuntu desktop, ssh-agent starts automatically only when you login at the console. If you login remotely, you need to say this:
eval $(ssh-agent -s) && ssh-add
If you too much ssh keys in your ssh agent:
ssh -o PubkeyAuthentication=no root@host