When lj' s are required I just wear the polyproplene lj' s and shirts, I use Helly Hansen and they are very soft/comfortable (no problems with " da boys" ). Cotton againist your skin is to be avoided as it retains moisture and when wet will become cool, yes this includes briefs, tshirts, etc. You' ll find that polyproplene underwear if you buy the proper size will fit or conform to your body, thus keeping everything snug
Adding more layers may help but if you sit and are damp at your core it defeats the purpose. You' ll find wearing a good blend of layers will allow to actually wear less clothes and stay warmer. Polyproplene garmets against the skin and layers of fleece or wool, then follow that up with gortex or similar fabric to stop wind and moisture from entering from outside(these fabrics are also breathable which is important to staying warm). Don' t forget to keep your head, neck, feet and hands warm as well.