Scoring in the post has a lot do with positioning and angles. If you can get a deep catch on your defender with a good angle as well, then there isn't very much that they can do. That is why the duck in is such a good move to use in the post. When doing the duck in move you want to go from slow and relaxed, to a violent quick move and catch the defender completely off guard.
This basketball training video is going to this basketball fundamental, and show you how to do a duck in to the baseline side and then finish with an easy layup at the basket. Really focus in on the footwork, timing, and positioning of the body when you watch this drill. A good duck in will cover all of those things.