When the boys hurt one another (even just hurt their feelings), I make them say they're sorry. And if it's a physical hurt, they also need to give a "gentle touch" as they apologize, like rub the spot on the person they hurt. This is a practice at their old daycare/school, and we incorporated that into home life, too. (We learned a lot of our current parenting practices from that place!)
Occasionally the practice has unexpected consequences however, like the time DS7 was 3yo and excited to see Daddy, jumping up and down flapping his hands. He accidently popped Daddy in a sensitive place, which sent DH to his knees. I blurted out, "Oh no, honey, you've hurt your Daddy!" so he reached out a hand and started to touch DH where he'd been hurt.
DH managed to grab his hand before contact was made, and say, "It's OK, honey, you don't need to give gentle touch!"