Back in the days when I used a Tri-top I occasionally wore a T-shirt underneath it. It didn't seem to make too much difference that I recall, but it did stop the binder chafing my nipples so much ;]

These days I just wear a Loveboat sports bra binder-thing with at least a couple of T-shirts over the top. It's very nice and infinitely more comfortable, but more fiddly and not quite as effective as the Tri-top.

I don't wear anything under my binder, but it is a double front compression binder and I am lucky enough to have a small chest, so it is usually all I need under what I wear. I like wearing baggy shirts most of the time, so my chest usually looks flat.