Usually my hands are down, one way or another. Sometimes, it's considerably more comfortable for my to cross my arms at my abdomen or chest, but that's normally only during the homily. For me, crossing my arms is my "concentrating" posture.
Either way - posture, shmosture! I think that people make a considerably bigger deal about this than need be. Different postures can mean different things under different circumstances for different people. If a person has a problem with someone else's posture when it's not being a distraction to anyone, then that person needs to get over him/herself. I spent ten years in the Navy, and "parade rest" (i.e., having my hands behind behind my back) is very natural to me, and, to me, is a very appropriate stance. I've heard that Russians say that having hands behind my back is "reaching for the Devil". That's ludicrous! No one's reaching for the Devil. Having my arms crossed helps me concentrate on the homily. Plenty of other people sit/stand with their respective postures for their own reasons. If someone stands or sits in a way that helps them pray, how is it anyone's business?