Paragraph 1: Take a couple of sentences here to explain your financial problem to your boss, being sure to put him in the picture with your personal situation.
Paragraph 2: In this paragraph you can expand on why you need to have a cash advance, and also state how much you will need. Another sentence or two can be used to explain why you should be granted this wish, and you can cite here previous good work, or examples of you going beyond the call of duty for the company in the past. Make sure that your request is just that – and not a demand for money. The ball is in your boss’s court, and you need to be polite in the extreme.
Paragraph 3: Thank them for their time and apologise for having to ask for a cash advance.