remember when you where young....your parents used to sew a piece of elastic between your mittens in winter and thread them up your sleeves??? well do this but with industrial strength elastic and make it slightly too short.... thus stopping any form of un-demanded hand gesture!!
or maybe it's your inner gay trying to escape and let the world know!!
Nah seriously don't worry about it...if people have got nothing better to do than point at others and pick fault, if their not your coach stick 'em the finger...all the hand actions and flapping will improve over time as you improve.
Take both hands and grab your jacket with them. Sides are best, though some prefer either the front (think above trouser pockets) or the back (above bum cheeks).