Yeah, I know exactly what you mean about the covering (very bad builder here). I'm extremely awful at it. The only thing I've learned to do is to pull HARD and move slowly. It helps to have an extra set of hands with you, just to do all of the pulling. Work slowly and pull in the direction perpendicular to which you're going to be working. Like, pull the Monokote towards the TE of the wing as you move your iron across the koting side to side, slowly moving your way down the wing. Coting just takes patience. I've found that the pieces I cover slowly, and with patience, come out millions of times better than if I'm even in a SLIGHT hurry.
I know it sounds basic, but most of it is all mental. Just relax, take your time, keep the koting taut and it'll come out better than you expected.