Using Pulverize can allow you to establish better positioning for Headbutt.
Movement speed is very important on Alistar. Consider which boots to buy carefully.
Using Flash can allow you catch your target off guard to knock them back into your allies with Pulverize and Headbutt.
Alistar is very disruptive but very tough - try to target more fragile damage dealers.
Watch out for the Pulverize-Headbutt combo when around turrets.
When Alistar uses his ultimate, it's often better to move back and wait until the effect wears off before attacking him.