The only real way to solve it is to use HTTPS. The Gmail team had this problem for the longest time and is one of the biggest reasons they added forced-HTTPS as an option in account settings.
Whatever you do, there will always be a non-trivial number of ISPs and company networks that have misconfigured proxies that overzealously cache sensitive data and display to everyone with not a care in the world.
Whatever you do, there will always be a non-trivial number of ISPs and company networks that have misconfigured proxies that overzealously cache sensitive data and display to everyone with not a care in the world.