Two options to all the girls wearing tie-sized mini-skirts on public transport (train, tube, double decker buses):
a) When you are getting ready to leave your house, think about the amount of stairs/escalators you are going to go up/down during your journey and, if you think you will be slightly embarassed, change into something longer or tighter;
b) If you don't have a care in the world about strangers/pervs/the whole world (scratch the ones you don't fit in) and still want to go to work with THAT mini mini-skirt, then behave normally, walk normally... whatever you do, don't freak out.
Because it may happen to you what I saw today: A girl that, when confronted with a flight of stairs at Green Park station, decided the best way to prevent people from seeing underneath (which was not that difficult already), was to start running towards the stairs, so she could reach the top before anybody could have had time to reach the bottom of the stairs. It would have been a great idea if she hadn't tripped on a step and fall down head first (and bottoms up!), leaving everyone behind her to see much more than just a glimpse of her knickers...
PS - I read this morning that the weather people guaranteed Summer in London was starting today again. My soaked shoes/trousers/shirt beg to differ!