You have to make an invoice. Everything dealing with money needs to put down on paper. It is a way of protect you and your boss (or client). Instead of keep calling him. Fax him an invoice. even tho he end up not paying you, you can use this invoice to the lawyor.
Remember, this is protecting both you and him. because only shouting out with the month isn't going to do anything.
This is what my dad teaches me.
Instead of putting it on paper afterwards, you should make a simple and legitimate contract before you even start doing anything. Discuss before hand about the payment details, and put those terms in the contract.
Freelance almost never get pay "right the way", in everywhere, because part of the reason they hire freelance is that they don't have to pay you right the way, and their client won't pay them right the way. They'd have to wait for their client to pay them, and then pay you. Big reputable company do this, (but with ensurance that you'd paied eventually unless they went under.) one man company do this. This is way they hire "freelance" instead of calling you a temp. Freelancers are operating their own business, you are your own business representative, they can deal with you in a business to business level. If they hire you as a temp, you're their employee, then you are protected under the labor law.
That being said, there's one reminder. If you start suing people and making a fuss about what's understood and accustomed about payment, you'd be sure you will have a very hard time in the future to find freelance jobs, at least in a certain circle. Freelancing is, afterall, about the "word of mouth".