How to Size Long Underwear Tops?

[Becky]

I should clarify... I'm right between sizes. If I want snug, I go down. If I want something more relaxed and casual, I go up.

[jessmarimba]

I'll second Becky - I'm also between sizes, so I size down for a baselayer. Though I always have to check length, b/c if a baselayer shirt isn't long enough to tuck in then it's pretty worthless! And then there's those athletic thighs... *grr*