How do I force my Mac to always boot Snow Leopard in 64-bit mode?

I was reading on Apple Insider that you have to hold down the “6” and “4” keys during system boot to enable the 64-bit kernel in Mac OS 10.6. You can find that info here: http://www.appleinsider.com/articles/09/09/02/inside_mac_os_x_snow_leopard_64_bits.html

Then I read on another site, 9-to-5 Mac, that there is a way to force your Mac to boot the 64-bit kernel every time. It specifically said, “use the NVRAM or the com.apple.Boot.plist file to more permanently boot into 64bit mode.” You can find that article here: http://www.9to5mac.com/snow-leopard-64-bit-32-bit-firmware-efi

With all of this said, what and where is the “NVRAM or the com.apple.Boot.plist file”, and how do I make my Mac always boot into 64 bit mode?