i've been in the same situation myself, to the point that I couldn't pay my rent for 2 months. Luckily my nexy emlployer was kind enough to give me an advance.
In all legal matters like this, collect all evidence of what you did and when you did it, and put it in writing. Make it clear what the problem is, how long it has been a problem, and what you expect them to reasonably do about it (like give you a $500 advance immidiately whilst they sort out the backlog).
Keep all emotion out of the letter, keep it to only facts. If you can provide a log of correspondence with them on the matter then also include this.
You need to do it this way because it gives you a record that they have been notified of the problem, and allows you to take them through court with ease if you need to (in which case it shows that you have been reasonable with them, and they are the ones at fault). In the UK we'd use the small claims court for this, and usualy the court summons (i.e. applying to the court for a hearing which only costs you £60) clears it all up.
I really can't stress enough that you need to keep emotion out of the letter, because it would only count against you if you have to take this higher.
