A lot of it has to do with the length and thickness of your hair. When my hair is long (past my shoulders) it is mostly straight as a board. But, when I have it cut short, it drives me crazy because it is so wavy and curly in all the wrong places.
When I was younger, if I wanted my longer hair to have some wave in it, I had to have it thinned to remove some of the weight of the hair so it would relax and curl. Now that I'm older and my hair is thinning naturally, I just keep it short and let it do what it wants to do.