Audi A8 (1994-2004)
Towards the bottom-right corner of the instrument cluster there should be a button labelled "0,0"

Start with the key in, but the ignition turned OFF
Press and hold the 0,0 button, while turning the ignition on (position 2)
Release the 0,0 button
"SERVICE" should now appear on the display
Press and hold the 0,0 button until the display shows "SERVICE IN xxxx Miles / Kilometres"
Release the 0,0 button, then turn off the ignition