I use a LowPro DryZone 200 Backpack. You can pack a ton of equipment into it, the shoulder straps and waist strap do a good job distributing the weight. And the thing that did if for me, is the waterproof skin that it has. I have been caught out in the rain a couple times and didn't have to worry about it. I normally carry one body with the Sigma 150-500 OS attached in it, and have room for the 70-200 F/2.8, the 150 Macro, 10-20, 17-50 Tamron and a couple others plus the flash and a hand full of filters/batteries/memory cards. And it fits in the overhead on planes without a problem.
Almost forgot, get the black/grey one, the yellow one stands out like a sore thumb 
--Evan


