I learned this early on in my backpacking days watching a older guy who had been all over doing it.

If you have problems on steep portage sections you need to be doing this if you are not already.
It doesn't replace being in shape but it helps either way. 

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Another trick is physiological. I learned this from the same guy. Never stare at the summit as you climb.
Just look far enough ahead so you don't stumble on any problem areas. Next thing you know you're at the top.