The best way to make people use it is to make it seamless. I am talking about something like a Chrome/Firefox extension, which creates a GPG key pair for you,

The best way to make people use it is to make it seamless. I am talking about something like a Chrome/Firefox extension, which creates a GPG key pair for you, uploads the public to some central server and encrypts stuff like the text in your hotmail, gmail, yahoo mail, etc, after you press "send" and before the text is sent to google. Then, at the other side, the a user with the same extension opens the new message and as soon as it appears, the extension detects that it is an encrypted message, reads the id of the sender and decrypts the message. There are lots of details which makes this difficult to implement... but that would be the only way to make the vast majority of people use public key encryption.