You usually have to be 'in' with the girls who do it. They'll usually have met eachother or been invited by another girl. You usually need to be good looking. All you do is stand around in T-shirts, low cut tops making the place look good, they'll occasionally make you do some promoting like handing out free drinks vouchers if they sign up to the club's FB page or something.
You get paid a lot to do it for very few hours' work.
Bottom line, you need to be good looking and know the people who currently do it.
Unless you're on about flyering or selling tickets, which in any case is easy and you just apply and get a job, your earnings depend on how hard you work and it can be time consuming and non-rewarding depending on how popular you are or how many people know your name and want to buy for events.