I don't have much advice here because ever situation is different and the sensitivies of asking is different with every organization. What's worked for me is being absolutely direct and honest in the situation and the urgency of needing pay. Often times the other side doesn't realize how much it affects you, and shattering that illusion is what's needed. In other words -- you kinda need to make them feel like an asshole for not paying you. Sometimes it doesn't work, though. At some point you will need to raise it as a legal issue and begin refusing work.

Good luck!