Mostly tippet to fly size. I won't go with lighter tippet if the fish are going to be smaller, since there might always be a big fish around. I pretty much have standard tippet sizes for situations, and seldom change them. I fished 3x tippet with #10 caddis in the Lochsa this fall, with water clarity in the 10-20 foot range, which is as clear as anything I ever fish, and they didn't care. In short, I will use the heaviest tippet I can get away with, so that I can land fish quickly and get them back in.