[Lewis geek]
Actually the wardrobe is only used in the first novel, The Lion, the Witch, and the Wardrobe. (Don’t believe HarperCollins’ lie that The Magician’s Nephew is the first volume or that their current numbering is Lewis’s original intent.) They return to Earth by passing through Lantern Waste and back into the wardrobe.
Here’s how Lewis transports his characters in rest of the* Chronicles:* I’ll put the rest in a spoiler:
In the second novel, Prince Caspian, the four Pevensies are yanked from a train station platform. There’s nothing special about the platform, but that’s where they’re sitting when they are summoned (either by Susan’s magic horn or by Aslan, depending on how you look at it.) Aslan opens a portal to return them.
In the third, Voyage of the Dawn Treader, Edmund, Eustace, and Lucy travel through a painting that reminds them of a Narnian ship at sea to the vicinity of the titular vessel. They are never actually in Narnia during this story, but rather in Narnia’s world. At the end of the story the three English children are in Aslan’s Country at the end of the world, and Aslan opens the Door in the Sky to return them.
In The Silver Chair, Eustace and Jill pass through the gate of the fence around their school into Aslan’s country. Aslan blows them on his gale-force (yet strangely gentle) breath into Narnia proper. At the end of the story, after now-king Caspian dies, Aslan takes him and the two English children back to the school by the power of his breath again, though it feels different; it seems to Jill that he blows the rest of the world away while the travellers remain motionless. In England Caspian is alive again, and he remarks that he’d be a ghost if he were in Narnia.
In The Horse and His Boy, there is no travel between worlds.
In The Magician’s Nephew, Digory and Polly travel in stages. They use magic rings (yellow and green) to travel to the wood between the worlds, wherein are set pools that can be used to travel to any living world. Aslan returns them by fiat to the WBTW at the end of the story and thence to earth. They bring with them a magic apple from Narnia, the seeds of which Digory sows. The new tree grows large but eventually is destroyed into a storm; its wood is made into the wardrobe from the first book.
And in the last book, The Last Battle, Eustace and Jill intend to use the magic rings again to make the voyage. However, they are killed in a train accident first (along with all the other English children in the novels, except Susan); they don’t realize that they’re dead, though (cf. Caspian’s remark about ghosts of people in other worlds). At the end of the story they travel through an enchanted stable into Aslan’s country, now explicitly revealed to be heaven; all other worlds are merely spurs of this greater land.
[/Lewis Geek]