Bill,

This only works for 1999 and later S70s (1998 requires a special tool):

With the engine off, insert the key. Then simulaneously press and hold the trip odometer button while you start the car. After the engine starts, the service light will remain lit for a few seconds, then flash on and off. When it flashes, release the trip odometer button. Voila! It's reset for 7,500 miles.

Note - if you keep holding the trip odometer button until the service light becomes steady again, then you will have done nothing (i.e. the light will remain lit). And, of course, this also resets your trip odometer - which may explain to you (and others who may read this) why your trip odometer has been mysteriously reset every time you went to Volvo for service.

Best,
George
1999 S70 - 151,000 miles and counting