I think practice is the key here.The more you try you learn how much tension is too much or too little.Sure you will mess up a few but eventually you will get it.I use Danvilles flat waxed nylon thread.I have tied many deer hair flies and all have been pretty ugly but even your ugly flies catch fish.The flies I tie now are not near as ugly as the one's I tied when I first tried,but even those would catch fish.I also tie a half hitch between bunches of hair.This is a great idea because you never know when the thread will break.Just my 2 cents.
-Steve