I usually do this on Cayenne's that are Canbus systems (I think 2005+)

- With key in off position, depress the ODO button.
- Without brake pedal depressed, turn ignition on while still pushing the ODO button.
- You should see a message: "Service Reset".
- Push and hold the "enter" button located on the bottom of the wiper stalk.
- While cotinuing to push the "enter" button, turn off the ignition.

Now you should not see the service in XXX or service now message when you start the engine.