From shulchanaruchharav Web site of HaRav Yaakov Goldstein shlita:
It is accustomed on Purim to allow males to wear the clothing of females and vice versa. This is not forbidden due to the cross-dressing prohibition as the intent is for mere joy [as opposed to promiscuity]. [Rama] [However many Poskim challenge this custom and thus practically one is not to do so.]