How do you teach your child to wipe?

[toooldforthisgame]

Not sure if this is the best place for this question as it's post-potty training, but how do you get children to clean themselves properly after using the toilet?
My son is five now, and we have shown him numerous times how to use toilet paper, yet he continues to use a small tree's worth each time but only gets a smear on it and then pulls his pants up. He's not applying pressure - is this common? It's like he just doesn't have the strength in his fingers/hand when he reaches round the back of himself.
He doesn't get how to blow his nose either, although his younger sister can do that pretty well.