I think this is the method I use on my 01 X5
(from another site)
Press and hold the left trip button in the instrument cluster.
Turn the ignition to the first positon (accessory).
Hold the trip button until the "reset" is shown, then let off, and immediately press and hold the button again until the "reset" starts to flash.
Let off the trip, and press it once more to reset.
2006 BMW X3 3.0i (mine)
-Black Sapphire w/Black Leatherette
2001 BMW X5 3.0i (hers)
-Jet Black w/Tan Leather