When playing Alistar mid, you out sustain your opponent. The way you harass in lane is by using your W and getting a basic attack off right before they get knocked back. Repeat that until about 25% health is left on your enemy, and rush in with your W + Q combo to knock and stun your enemy and just beat him down from there. (Note: NEVER HARASS WITH Q! Q is only for securing kills/escapes)

Since Alistar offers a huge amount of CC, if you go to gank a lane, it's a secured kill or force flash. If you're going onto an enemy with your teammates, you should use your W + Q combo so they can catch up. Alistar ganks lock down the enemy really well, this is why i take mobility boots over sorcerer shoes. Alistar has amazing utility.

TEAM FIGHTS:
Although you do a lot of damage, you should always peal for your adc. In team fights head in with your ult so minimize any incoming damage.