i am training people to use Thunderbird + Enigmail + GPG since few years. i see sometimes some of them occasionally forget to encrypt a message. sometimes it may have really serous consequences (for example for the human rights activists in some parts of the world). i was thinking how to make it easier for the people to see that the message will not be encrypted or will be encrypted so they would not forget.
one idea is to make it much visually obvious that the message is encrypted or not.
for example to have a coloured border around the email compose window in Thunderbird. say red = you not encrypting the message, green = message will be encrypted.
or to have a big watermark-like icon over the background of the compose window in TB, say eye = message will not be encrypted, key = message will be encrypted.
another idea is to include the option so TB will ask you each time you send the message 'do you want to encrypt/sign this message?'