Originally posted by Al_Kahollick I'll second the recommendation for the Pentax hand strap. It works pretty well. I've used it on a K-5 and now a K-3.
For your existing hand strap, a cheap fix to the slipping problem might be as simple as wrapping a rubber band around the strap's base so that when you attach the strap base, the rubber it is pinched between the strap base and the camera bottom. The rubber, being a little tacky, will make it less likely to slip/rotate out of position. This is a trick I used to use when I had a cheap tripod mounting plate that wouldn't cinch up tightly enough to the camera base to keep it from slipping with the camera held sideways (in the portrait position) from the tripod with the somewhat heavy DA*50-135 mounted.