Adam Dachis at Lifehacker has written a cool article about techniques for figuring out how best to think about a problem you need to solve. It bugs me that he uses the language of thinking "with your head or your gut," though I understand that it's the language most frequently used around this kind of dilemma. He refers to reducing head-thinking as "Stop Overthinking," and reducing gut-thinking as "Ignoring" your gut.
That biological distinction is nonsensical. All thinking happens in your head -- and there's no such thing as overthinking. There is such a thing as thinking poorly, and it's very possible to go quite a long way down one train of thought when a few steps down a different path would have led you to the answer. But it's not like, when you do that, you've come up with the solution then kept going. You're not thinking too much, you're thinking wrong.
That said, the article offers some pretty cool techniques for dealing with wrong thinking. Dachis suggests that, when you're spending too much time actively contemplating about something, you should get in the habit of asking yourself (about everything) "Does this matter?" And, if not, don't bother. For instinctive decisions, he suggests framing the decision in your mind as an offer by a mysterious stranger of what you'd lose if you made the decision -- I know I'd probably do better letting my gut decide whether I want to keep $384 than I would letting my gut decide whether I really want that incredibly sexy paisley tie.