Air-time, it turns out, is that momentary inertia you feel when a coaster perches on the top of a big dip. You'll want to slow your cart down before the peak to get big air-time scores.
You can boost your scores by building scenery near the track, using tunnels, bending your track into sharp banks, and fixing sections where crazies fly out of the cart.
Change the coaster and track type (tap on the rollercoaster station) to build rides that earn money more quickly, or have a higher cap on maximum profits. Get a mix of the two to make the most money.