My mother was insistent that my siblings and I knew Santa wasn't real. She was very naive as a child, and when she was 16 someone at school made a joke about kids that believe in Santa. She got in an argument with them, herself believing that Santa was real. Kids were cruel, as they are, and I guess it followed her for the rest of her school days.
My own children are young right now, so I let them enjoy it. I'll just make sure when they get a little older they know who's really buying those presents.
I figure the best thing to do if kids are getting older and still appear to fully believe is to start dropping hints. For example, Santa is usually last to go after things like the Tooth Fairy and the Easter Bunny, so you could mention them together. Eventually the kid will figure it out on their own, which is a lot more pleasant than just being told one day.
My own children are young right now, so I let them enjoy it. I'll just make sure when they get a little older they know who's really buying those presents.