Have you tried the front-tuck yet? It's one of the best (and easiest) ways to create that undone-done look that makes it look like you're not trying too hard.

Here's how you do it:

Pick a t-shirt or button-up shirt that fits you well and that hits about halfway down the fly of your pants.
Button your shirt all the way to the bottom. (Even though the front will be tucked in, it's easier to control if it's buttoned).
Tuck the front of your shirt into your jeans or chinos (again, these should fit well).
Don't think too hard about what you're about to do next.
Pull the front out so that it's blousing over your pant waistband as in the image above. The front/center 3/4 of the shirt should stay inside your pants.
Go on with your day! The less you fuss around with this look, the better.

Note: this does not work with baggy shirts (it just looks sloppy), so make sure you don't skip step 1!

image via people.com