Tip 03: Move along, there's plenty of room (1)
This tip follows on closely from Tip 2.
Once on the train, there is no need to stand in the doorway. If you entered a building, would you stand in front of the door? (Actually in London there are quite a few people who seem to enjoy doing this too...)
Move along, there's plenty of room for standing between the seats.
It's incredible the number of people who refuse to move further inside the carriage and cause people around them to have to cram in around the doors.
It's actually quite funny sometimes watching such groups reach their critical mass, at which point the stubborn "I'm not moving down the carriage" guy finally has to give in and move along in order to breathe.
More often than not, your journey will be more comfortable if you move away from the doors and stand in the aisle, if for no other reason than not having to have your nose shoved into the next passenger's armpit.