I wear tank tops under everything. An important leggings rule, wear something long enough to cover your butt/crotch. A long tank is a great option under a t-shirt or shorter top. I also wear them under my t-shirts with jeans or other pants as it prevents skin showing if a t-shirt rises and smooths things out a bit.
I'm on a never ending search for great tanks, tight fitting, smooth fabric, long. Some of my current ones need to be replaced so I decided to try Spanx. Women everywhere rave about this stuff and I figured it would be good at smoothing things out.
Soooo...why do women wear this crap? I felt like a sausage. It sure smoothed things out, but I felt fatter wearing it than I did not wearing it. The interesting thing is they have no compression added in the bust, in order to "show off the girls" as they put it. I'm all for that, however walking around feeling like all the fat from my stomach and hips is going to suddenly shoot up into my face isn't worth it. And I thought I'd worked off a lot of my fat. Ummm...nope!
Now Spanx, design one like in the cartoons where the extra fat can shoot up into my breasts and I'll be sold!