You know, I think there's a trick to this, and I think I stumbled upon it totally by accident.
First of all, I think what Suz says is true: this is easiest on slender individuals with long necks and not much in the way of a bosom. I don't fit this description exactly. I am slim and have small bone structure, which makes this look easier, but I also have very square shoulders, a short neck and not a tiny bust (although I would not really be described as buxom -- but still, they are there, you know?), which are all features that make this look more difficult.
First: shirt must be tailored, and fit well. I don't tuck mine, since I prefer the look of the shirt-tails hanging out below the sweater hem. The fabric can't be too soft or floppy, but rather fairly structured. A Crisp White Shirt.
Second: leave the top two buttons undone.
Third: the sweater must be the right weight, and should drape nicely. Elbow sleeves help. I stumbled across J. Crew's Tippi sweater -- fine-gauge merino wool with a round neck and 3/4 sleeves -- last year, and it's perfect. I can't imagine a cotton sweatshirt would work, as it would be too bulky and stiff.
Four: roll up the sleeves of both button-front shirt and sweater. Cuff the shirt over the sweater. Again, elbow sleeves help (functionally less bulk). There will also be visually less bulk with rolled-up sleeves, and it does something magical for proportions. (I roll or scrunch my blazer sleeves too, for the same reason).
Might be worth a try. It might work on your figure too!