Macbook Air Remote Install driving me crazy

Hey,
Im trying to do a remote clean install for a Macbook Air that I sell at my work basically so we can sell a display model reduced.
Ive taken it home to try a remote install of my own macbook and the process is driving me insane!
I finally figured out turning off the WEP protection on my wifi network and also turning on dcd/cd sharing but every time i choose the Mac OS X install disc, the Air waits a few minutes and then boots up normally. I phoned apple and they told me to use a firewire cable???!!!??? which wasnt much help

anyone have any pointers or tips.
im follwoing the standard airport install instructions sent by apple

Cheers

Gmacp

Start up holding the option button. It will then give you the option to select the startup disk.

Im doing that, its only after I choose the start up disc does boot in normally