In the short time started CCing, I noticed that a thin, but lightly starched dress/sport shirt covers a big gun very well, much better than some relatively heavy t-shirts or sweaters, because the stiffness of the fabric keeps it from following the contours of the gun.

I've been carrying a full sized, 1.5 inch thick Cougar lately, and by far, it gets covered best by an untucked dress shirt.

Just thought I'd share. Maybe others knew this or find it obvious, but it's new to me.