Lots of good advice already. Form can certainly affect string slap. So can bulky clothes. When I really bundle up, I do a number of things to avoid string slap. I incorporate a vest into my outer layers, obviating the need for one jacket/shirt, etc. with sleeves. I also use an extra long arm guard, one that starts at the bicep, bends at the elbow and ends at the wrist. I might also put a little extra bend in my bow arm. Opening my stance a little, i.e., turning into the shot a bit more, also adds string clearance. Any one of these helps. Use all of them and you won't hit your clothing. Good luck.