M.H.
I am going through the same thing with my son. He started when he was about 15 months old, got better on his own, and now at almost 20 months old is at it again. I'm sure it is just because of his lack of verbal communication (i.e. "that bother's me please stop"), so he hits. We have been trying a few things and the stuff that really works is first telling him no. If he does it again within a few seconds we grab his hand and tell him that it is not nice to hit people and stop (firmly of course), if he does it a third time he goes to time out (which consists of sitting in his crib for anywhere from 1-5 minutes depending on how angry he was when he hit). It is amazing how even just a minute by himself in his crib makes him think about things. Without fail, he is calm and controlled when picked back up. Good luck...I know it's a challenging thing! :)