It doesn't need to be straight off the boil, as long as it's hot, that should do the trick. Just leave the brush in there for about 5 mins, then take it out and pinch the bristles back into shape. You can wrap it in some cling film to hold the shape while it cools down.
If that's no good, I know a lot of people run the bristles through some clear base coat, shape it and then just wipe it on a lint free wipe, it's fine for some base coat to still be left in the bristles, that will help it keep its shape in storage. Keep it's lid on, or if it hasn't got one, you can wrap up some tinfoil for a makeshift lid.