Short answer, you don't. Use SSL.
Longer less desirable answer: Client Side Crypto. Essentially forming your own SSL (crypto wise) using PKI (RSA) to share a symmetric key then sending the password over encrypted(AES) with said symmetric key.
This could still be worked around because you're missing the Certificate Authority part, and a few other things but it would stop the average fgt with wireshark.
Have I mentioned SSL?