64 bit structure alignment?

I've just started working on a port of a driver from 32 bit to 64 bit XP and
I think I'm having trouble with alignment in my structures. For example, a
UNICODE_STRING is two shorts and 32 bit pointer. Using WinDbg, I can see
that a pointer to a UNICODE_STRING being supplied by the system to a call
back in my code is using two shorts, four padding bytes and then an 8 byte
pointer to a wide character buffer. My code is not reading this structure
correctly. What settings should I be using at compile time to get my code to
properly interpret structures?
--
Rick