My gear is too heavy. I had a shoot lasting only a few hours and the next day I experienced thumb and wrist pain.

Canon 1Ds - 1,600g
Carl Zeiss Vario-Sonnar 17-35/2.8 - 900g
Sunpak 120J flash - 522g
Total approx 3kg or 6.6lb

I heard that the supplied hand strap helps tremendously, but most of my shots are in vertical orientation and the strap only works in horizontal shots.

Are there any products available which would allow me to use the strap in vertical orientation?

I know those of you who are wedding photographers use even heavier stuff all the time. Any tips you would share is greatly appreciated.

It's that flash at the top that does it. Throws everything off-balance. OTOH at least it's not the 70-300 .

I bought the hand strap when I got my 1Ds2 three years ago and frankly couldn't get used to it. I never actually used it. You can have it if you want it, though the shipping might be too much even if the strap is free ...

BTW, thanks again for the 17-35 EOS mount. Here's one from Monument Valley a couple of weeks ago, 19mm:

Lovely late afternoon ...

My secret doesn't help you - I use a tripod 85% of the time, though that means I then have to lug the tripod too. I'm just glad I never use flash.

Cheers!