wow, suprised by all the anti-thinking talk 'round these parts. me, i like thinking (i think). i think a lot. though i do agree with backjob on the thinking = insomnia part.
but what is thinking? how do you do it? a tuff question. Maybe the simplest answer is just sort of an inner monologue with yourself. Talking through things in your head. Though it's not quite like just talking it's more complicated than that, it goes a bit quicker, there are leaps between thoughts. But nevertheless that's more or less my subjective experience of thinking, just talking things through. Sometimes when i'm considering some more abstract concept, i imagine myself trying to convince someone of my case, and try to think of what i would say to convince someone.
I'm guessing this is what you people mean by "thinking": a sort of conscious deliberation on something that is not directly related with what you're currently experiencing at the moment. Because of course an argument can definitely be made that as long as your awake you're doing some sort of thinking, conscious or otherwise.
Another interesting point (for you non-thinkers) is the theory espoused by some cognitive psychologists and such that your conscious mind really doesn't do most of the thinking anyway, that the vast amount of thinking is done nonconsciously, and that all the conscious mind really does is kind of create a narrative for all that has gone on, a narrative which may or may not accurately reflect what actual nonconscious thoughts occur. So conscious thinking might be mostly bunk anyway...