There are two things I would suggest, the way you grip the bow and how tight you hold the bow. Try gripping the bow with your hand in the position it would be in if it were picking up a suitcase. In other words, the further you rotate your hand into the bow, the more likely your forearm will get slapped. On the second point, you need to reach a happy medium between gripping the bow too tight and holding it so loose that you grab the bow on release. Some of us solve that by holding the bow with only our thumb and forefinger wrapped around the bow, and the other fingers loose to the side. If that fails, well, I guess that's why they invented arm guards!