Always care/bring emergency cosplay supplies. Needle and matching thread, tape, glue of some kind, etc. I'm always guilty of forgetting repair supplies and then something breaks and I go "Crap, why didn't I remember to bring x?"
Also, think about poses while at home. Nothing sucks more then derping out when someone asks for a photo and you can't think of anything other then standing there.
Those are my two big one's I always personally forget. Shame on me.