Like I’ve said before, your body will talk to you a lot when you meditate.  My body talks a lot louder than my thoughts, and even though I know it is a good idea to be “in touch” with you body, sometimes I miss the times when I had no awareness of it at all.  Then again, maybe some of that is just that it gets louder as you get older, whether you meditate or not.  I usually position my hands either upright on my thighs, or palms down on my thighs, or sometimes with my hands cupped in front of me.  More than once, I’ve suddenly noticed that my hands have clenched into fists, the exact opposite of the relaxed, intentional posture you’re looking for.  I don’t have any particular wisdom to share about how to prevent it, just notice and relax.  What I can tell you thought, is that the more often I catch it in meditation, the more often I catch it during the rest of my day – finding some part of me that is clenched tight for no apparent reason.  I take it (in a purely non-scientific way) as an indication that my fight or flight response has been triggered, or in my case, spends most of its time triggered, and that’s just not sustainable, at least not without consequences.