People ask me how to layer shirts under a sweater. It's really a habit: you either get used to it or not at all. Personally, I think the thinner the undershirts are, the easier the job becomes. However, the outter layer is very important, too. You don't have to go 2 size up in order to tuck something underneath, but keep an eye on those sweaters or tops that have good drapy feel and also some sort of stiffness. This way, you don't end up looking like a hot mess. :) Hope this helps a bit.