I find my sole T-shirt (or, more often, work shirt) bra useful, but then my breasts look cartoonishly large and pointy whatever I do.

Do you have a sports bra? They squish your breasts together and clamp them (to me) weirdly tightly against your chest, but you might have better luck getting a smooth silhouette with one of those, and obviously they're designed for activity. And not padded.