I've got that OpTech strap Ben recommended on my RB67.
It's very nice for what it is, but I rarely use it--I found I didn't fancy the weight of the huge camera bumping into my chest on the end of a strap, and that it tended to get in the way.
I normally use the camera without the strap. But another nice addition is the L-grip with trigger, which makes handling the camera a bit easier.
Still, if you really do want a strap, the OpTech is the way to go--well made, comfortable, fits the Mamiya lugs perfectly.