Short answer: use HTTPS.
If your site doesn't support HTTPS, use "ssh -D" over Tor or VPN over Tor
to make encrypted ssh tunnel.
This is a bit more difficult since you will need an anonymous SSH server (which you will never connect directly, and which you buy for bitcoins or cash or some other anonymous way).
But if your site supports HTTPS, then the life is much easier. All traffic is already encrypted, and even if Exit node tries to unencrypt it (by certificate replacement), Firefox will give a Security warning.