One way to make good money is airport fares, as they are usually long rides with big tips. The problem with them is that at most major airports, the wait in line, which is a highly regulated affair, is quite long.
Other options (hotels, bars, grocery stores) involve less waiting but are shorter rides with smaller tips.
Ah, the eternal cabbie question: airport or city? I vote for the city...quantity not quality.
I'm with you. Waiting for hours at an airport, and even then possibly getting burned, didn't work for me.
As far as making money goes though, "runaway" hooker/junkie fares from bars/clubs at times left me just as broke.