LeeD said..
To keep a waist harness from riding up, just lower your booms.
When hooked in, if your booms are REALLY low, your waist harness will be forced to slide DOWN on your body.
Do a compromise and have the harness lines perpendicular to your body when hooked in, and the harness will stay IN PLACE.
True,however sometimes you need the boom high for light winds/earlier planning and leverage,what ever is the most comfortable I reckon(as per Brett Morris comment)
Like all harnesses(and most things windsurfing) it is trial & error till you find one that fits perfectly and is comfortable for your style.