When you see a hot chic and you cant get yourself to walk up just start picturing yourself as an old man, and how you would look back and regret not walking up. Fast forward your life a bit, and you will see that minutes after she tells you no, its like nothing happened, versus the potential reward if she says yes.
You must fuck with your brain a bit in order to walk up (obviously some guys can walk up no problem).
Its a big huge numbers game, just do it.