Using cones correctly is definitely important, as Jas76 said. I still remember being in college and buying silicone serums like Frizz-Ease and basically POURING them on my hair, expecting a miracle, and wondering why it didn't work and just looked terrible. My hair is medium to fine and thin and gets weighed down easily, so that was pretty much the stupidest thing ever. I cringe thinking about it.
*3B, medium/fine, normal porosity, normal elasticity, shoulder-length when dry.