Generally, (as a female) I would give my seat up to the elderly, pregnant or disabled instantly, and try to aim a pleasant, friendly smile at the others waiting (very hard at times, I know) to let them know that you have acknowledged them, but that you have given your seat up to those you have chosen to.
Also remember that YOU have a right to a seat while you are waiting.
You are entitled
(Repeat this mantra please)
Some people may appreciate you giving up your seat, others may not - everyone's different, and I think at the end of the day, most people are just to be attended to - not nab the best seat in the house, like they are attending the cinema (if you understand what I mean)
I would just stick with the above 3 mentioned for now - elderly, disabled and pregnant - everyone else should have gotten there first
Great question by the way