Air layering isn't so much a way to propagate plants as it is a way to revitalize them.  Through air layering, top-heavy, woody-stemmed plants are induced to grow new roots higher up on the stem.

You can air layer schefflera, dieffenbachia, ficus, and dracena species that have lost their bottom leaves.  Use a sharp knife to make a cut approximately one-third of the way down the stem; make an upward slit, cut a notch out of the stem, or just scrape away enough of the bark to expose the plant tissue.  Don't cut more than halfway through the stem.  If you simply slit the stem, hold the cut open with a toothpick or matchstick.

Next, wrap a baseball-size clump of sphagnum moss around the stem where you've made the cut; wrap with plastic and secure with wire twists or string.

Periodically check the moss for moisture; if it has dried out, mist lightly and rewrap the plastic.  In several weeks (or months, depending on the species of the plant), you'll see roots forming.  When they fill the plastic wrap, cut the stem off below the new roots; then plant, moss and all.

The leafless stem can be left to sprout new shoots, or in the case of the dieffenbachia and some of the dracena and cordyline species, the stem can be cut up and planted.  New plants will eventually grow from each of the cuttings.