in my observations,
Humidity (in general) will promote aerial rooting,,,
i once enclosed the bottom 1/2 of my f.retusa (pot & plant) in a large ziplock bag (partially sealed), and it encouraged aerial rooting,,

another 'trick' bosai-ists utilize is to insert (the already started root) into a plastic drinking straw, 'aiming' it towards the soil-line where you want it to grow.

you could try hanging moss from the branches, and spritz it frequently,,
or pending the plants overall size, enclose the whole plant in an large clear jar, (pretzel, cheeto's, jug, whatever,,), or aquarium, maybe,,?

..but be mindful of rot, mold, fungus etc in in overly constant humid atmosphere. :)

.... agree that high humidity and good vitality are the keys to adventitious root growth. If your plant is small enough, you can put it in a 10 gallon aquarium (or a plastic storage container) with a glass or Plexiglas cover that allows some air circulation around it, and a 24" 2-bulb fluorescent fixture (cool white bulbs) on top of the glass & a propagation mat under the container. Keep a small pan of water inside the aquarium with the plant. The warmth, bright light, and high humidity will pay off in the form of plenty of growth, including adventitious roots.

Al