I've been doing USER@INSTANCE.sde but I'd like to know what "standards" everyone else uses.
USER@INSTANCE.sde is a very common format from what I have seen and I use this format as well.
I also use USER@INSTANCE.sde
I have started using
prod_ or dev_ tacked on to the front of of the email@example.com Just so I know what server I am on.
I do something similar for ArcGIS Server connections.