Straight hair with volume??

Hey, I have kind of wavy hair, it's pretty thick. I usually straighten it, but when i do i find it goes flat. My roots have no volume at all, well the bottom poofs out a little more, making it look like a disgusting christmas tree XD any styling techniques I can use to get nice volume all over my head (espeically my roots!) with my straightening iron? I already use herbal essance volume shampoo, but are there any other products you could reccomend that aren't expensive?