Two iPhones on separate accounts I got a new iPhone, and I want to give my old one to my son.  Naturally, I want to disable my Apple account and iMessage on the old phone. How do I do this without affecting my own account?

My son has his own iTunes account and will be on a different network than me.  The phone is unlocked.
The best way is to go into settings/general/reset and select Erase All Content and Settings.

This will reset the phone to an out of the box state, and allow him to set it up with his account without any of your data on the phone.