I'd like to say all of the advice here is good, but I can't.

We use a payroll company to pay all of our crew and talent. After the invoice is submitted (usually the same day as the shoot unless we're on a remote location) it takes 3 to 5 working days for the checks to be cut and delivered.

When the average shoot costs us $15K in salaries for talent and crew, it would be INSANE to have that much cash on hand in a location full of people we hardly know.
Sorry, but we run our business like a business (and that includes reporting subcontractor wages via 1099) and all I can say is if a business screws you, either take them to court or don't do business with them ever again.

But telling girls not to work unless they are paid up front is ignorant. This is a legitimate business and not street prostitution.
I certainly don't get paid for work I haven't performed yet and I usually don't get paid for work I have performed until the end of the pay cycle (twice a month).

On the ONE occasion where we had a problem with a girl, we told her to hold onto her model release and bring it in with her when the check was ready. Without the model release the scene is useless to us so it was her way of insuring she got paid. She has since worked for us on a number of scenes and pay was never an issue again.