Domke FA-140 1.5" Wide Gripper Utility Bag Strap Black. Review:
If you need a 1.5" strap, this is the one to get. I got one of these, now I want one for any bag I use that has a 1-1.5" strap.
MUCH more comfortable than thin nylon straps, and the "gripper" really does work well. Keeps it in place, but if you want to slide the bag it will also let you. Also has strong metal hardware and great adjustment range.
This strap just feels great, get it - you'll LOVE it.