I know what you're thinking - I don't need shapewear. I thought this too once, mostly as a refusal to admit that I don't look like I once did and with the opinion that serious underwear is for old ladies. However, one day I noticed how every third woman's bra was visible through the back of her top - even the skinny ones. I checked out my reflection in the bathroom at work, and sure enough, mine was too since I was wearing a fitted tee.
Off I went to Nordstrom Rack and I picked up one of these - a tight, seamless tank top that smooths everything out and hides the bra (mine was $22). It doesn't bring me down a size or make me uncomfortable during the day, but makes a big difference when I'm wearing something thin.