I use my hands, sand paper, filler, and the sun (no kidding). If you look at the sun with the helmet in between, you will see the asymmetry, then just use filler with it. I'm actually working on a helmet and that's what I do.
Also, before molding it, keep the helmet in your room for a week or so. When the week is over you'll see a lot of things you want to change and then those things won't bother you on the final casts.
BTW, remeber, the perfection is the asymmetry, but it needs to look symmetric, just like the Stormtrooper helmets or any other Star Wars helmet. None of them are symmetric, but they look like they are.