My transition to a professional wardrobe has been fairly recent, as has been my transition to a professional person (albeit one who's generally not very professional). When I was younger, I truly thought I'd have button-downs of every shape and stripe (organized by sleeve length and color, of course) hanging in my closet. Apparently, my former non-professional t-shirt wearing self wasn't having any of that and latched on pretty hard to the "professional tee" paired with trousers and topped with a jacket or cardigan. I recommend it.
$19.99 (down from $59 - you should definitely check out the NW sale section - crazy reductions all over the place!) at Nine West