We use full body harnesses by Yates (
Rope Rescue Harnesses, as well as adjustable pick-off straps as tethers. Generally, the photographer is tethered in two places, middle of back and hips. The middle of back tether provides adjustability for lean, and the hip keeps his butt in. We generally work out of AStars and TwinStars with half of the seats folded up, and the photographer sits at the edge for maximum field of view and better ability to shoot straight down. The Yates harnesses provide several D-Rings that camera straps and other equipment can be attached to for safety (just looping the strap around your neck is NOT a great idea...). I do note the suggestion that camera gear should be tethered to the aircraft, not the individual... but that's not always practical.
I haven't found a quick release system that I'd trust not to get fouled in our gear during camera changes. Both the photographer and assistant carry a small seatbelt cutting tool in an easily accessible place on the harness. I'd be wary of using a simple climbing harness if working at the edge, as well as using the crewman safety belt as the sole restraint if working with the door removed.
All camera equipment (bags, gyro battery, etc) is tethered to hardpoints using quickdraws or longer webbing. For me, AStars have the easiest to access tether points, using the rear seatbelt attachment points. Bell 206's are a little bit more of a pain to access solid points on which to attach carabiners, but small bits of webbing around the seatbelt attachment point piece works for them. Interesting is John Eacott's mention below that seatbelt mount points may not have appropriate lateral strength. Mechanics in the past have advised me that seatbelt points were appropriate for this usage, but I'll have to revisit that.
In agreement with TRC's point, anything loose MUST be removed. When operating doors off in 350/355, the velcro on the rear cushions can come loose in the wind in certain conditions and create a dangerous situation. Also, the photographer should have the lens hood taped to each lens as the twistlock can come loose easily. Lens caps and things of that sort don't need to come up.