I have never attempted to earn rays in rlc, I have however spent a lot.
From what I can see there is the hard way to earn, meaning you really have to like it and be good at it, deco and clothing design. These both take an enormous amount of time to learn and be proficient enough at to actually make any profit. Less technically difficult is the spammer, greeter, dancer type occupation. This requires you to amass a friends list long enough to fill an event quickly in order to be in demand. Lastly, that I know of, depends on what you will and will not do, i.e working girl and slave auction. This last category I know little or nothing about. I presume however that it is the easiest and least time consuming way to earn, however I imagine for some it may prove to be the most demeaning as well.
Oh and I forgot DJ, that is from what I can tell, a good way to earn. You of course have to have the equipment(what equipment? I haven't got a clue. lol), HUGE playlist, and raw talent to be any good at it.