I'm cheap, and like to keep it simple. I just wear a bandanna under my helmet. I fold it in half, throw the triangle over towards the rear (also protects my balding head from sunburn), and tie it loosely in the back.
The only time that I have problems with sweat in my eyes, is when I'm on a 3+ hour ride on a 95+ degree day, or if rain water exceeds the absorption capacity of the cloth.