How do Lucy and Edmund get into Narnia?

In The Lion, the Witch and the Wardrobe, they entered (with Peter and Susan) from inside the wardrobe. But the entry wasn't there every time. In Prince Caspian, all four children were pulled in by the blowing of Susan's horn from inside Narnia. In The Voyage of the Dawn Treader, they were drawn in (with their cousin Eustace) by looking at the painting of a Narnian ship.