Developer questions

Hi guys!

I have a couple of questions about offline business.

I'm starting my second company. It's going to be a development company focused on mobile devices.

When I get in touch with clients, and I start developing the programs - how do I charge them?

Do I do payment per stage of development? Meaning; When I finished 1/3 of the program - I send them an invoice 1/3 of the sum?

Or, do I send them an invoice when the project is 100% finished?

Thanks

I'd invoice them for half upfront, half when you finish.