I am an extremely sweaty person...I wish I weren't, but I am. About a half mile into my morning ride into work I am already sweaty no matter what the weather is like. The best cure for the helmet hair I have created a 2 part system.
#1 I wear a bandana/headband or other type of covering to keep the hair off of my sweaty forehead. I use a "buff" that I've had for backpacking for a while now (
http://www.buffwear.com/buff-headwear/original-buff). I suppose you could make yourself a similar moisture-wicking fabric tube at home pretty inexpensively if you have the right equipment.
#2 Baby powder. It sounds weird, but it works. I dust a little bit on the roots of my hair in the bathroom before work, rub it all around my scalp, and fluff (and possibly brush through if I'm feeling fancy). I have dark brown hair, so it took some getting used to in regards to how much powder to apply. It works wonders in getting rid of the pasted-down, sad, limp hair.
I have it down to a 2 minute pre-work routine at this point. I just leave the buff in my helmet and put it on before each ride. Each step does work pretty well on its own (and I will most likely just wear the buff if I know I don't have any clients coming in that day)...just a matter of trial and error. Hope this helps!