Saying sorry. I say it when I shouldn't (very Canadian of me) and it's for innocuous things but I have a hard time saying sorry when I really should. Like when I piss off my wife or if I am too hard on my kids about something they did.
I am getting better at it though. Admitting you are wrong is a healthy way to live and I always want my kids to be honest with themselves and us when they mistakes or do or say hurtful things to others. So, since I am trying to set a good example for them I make my best effort to apologize when I have done something wrong, even if I really don't want to admit I was wrong. Haha