One on from what EspressoJunkie says, when you go into the menu you can select the mode for in-built flash (TTL, M, --).... set it to '--' then press the button that okays this selection ('enter' i think).
What '--' does is sends a signal from the pop-up flash to the speed light but it shouldn't contribute to the exposure. It all happens in milliseconds so to the naked eye, loooks like the pop-up flash is going off as normal. If you do feel there is a contribution for the flash, you can get an SG-2 diffuser that cuts the power of this pre-flash signal