You can use this to your advantage by using Pulverize followed by Headbutt to push them into your turret's firing range. Be aware that skilled players will know this gimmick and will not fall for it unless they are caught unaware.
Using Pulverize immediately after a successful Headbutt will apply the stun from Pulverize. Instead of being pushed back, the target will be knocked up and stay in place. This can be useful in some chase situations (make sure you succeed or you will knock them further away), or finishing off a weakened opponent on the run. This tactic is risky, leaves you useless while both abilities are on cooldown, but is considered a staple move of Alistar and used very often in both competitive and casual play.
In the event that a tank is running close to your target to protect them, a safer and often more useful strategy is to use Headbutt on the tank to get you into range to Pulverize your true target.
Be wary when using Headbutt because you might end up disrupting positions in a way that benefits the enemy team, particularly knocking them out of a teammate's area of effect.
Alistar is considered a great roamer due to the fact that all he needs is his Pulverize and Headbutt combo to gank a lane.
Hitting an enemy into a close-by thick wall with Headbutt will stun them for ~1s. Reason being that they will still try to travel the distance even if a wall is in the way. Thus they will be stuck against the wall for the duration of the flight.
However, if Headbutt is used near a wall, it sometimes sends the enemy onto the other side allowing them to escape.
This can also be used to your advantage. Sending the enemy tank or a high damage dealing carry (like Fiddlesticks after he uses Crowstorm) over a wall can allow your team to win a 5v4 match.
Most walls in the jungle and river are thin, but whether they will go over them or not depends on how close they were to the wall at impact. If they are a high priority target, it may be better to keep them in the fight. A high priority threat would be neutralized if knocked over.
Unbreakable Will should mostly be used when turret diving or in team fights when you are being focused.
When using Headbutt you are always guaranteed an auto attack with it. This allows your Headbutt to proc items such as Sheen. Be careful when using aHeadbutt/auto right after using and auto attack, for the auto won't be guaranteed.
When you are in your turret range, consider staying close to Alistar so that if he uses his Triumphant Roar he will draw turret aggro because of his passive damage from Trample. But be aware that would make you an easy target for a Pulverize->Headbutt combo.
Flash can be used on Alistar to quickly get into the middle of a team, hit much of them with Pulverize, and knock a squishy target toward his team with Headbutt. Always spread out when you see Alistar walk toward your team.
Beware the potential combo of canceling the knockback of Headbutt into Pulverize. At that note, it is safest to poke Alistar with something that is farther than the 650 ability range of Headbutt.
Alistar is a pure tank that naturally has a high health, armor and magic resistance. Buying items such as Last Whisper or Void Staffcan help with killing him faster.
It is best to wait until the duration of his Unbreakable Will wears off before continuing to attack him.
If Alistar has already cast Pulverize and Headbutt, consider ignoring him if possible for the cooldowns of those two abilities are very significant - especially since spending effort to attack him may just have him reduce it with Unbreakable Will.
Due to said significant cooldowns, crowd-controlling his own team as he uses Pulverize if possible can waste the ability's synergy with his allies and leave Alistar with little else to do for his team while it cools down - as long as you yourself have other things to use while you traded cooldown(s) of your own for his, of course.
Avoid fighting near minion waves to ensure that his Triumphant Roar has a long cooldown.
Position yourself in such a way that it is difficult for Alistar to Headbutt you into a bad position that may endanger yourself.
However, be wary not to get in range of Pulverize instead. Try to position yourself properly and keep distance at the same time.
Flash or other movement abilities (such as Arcane Shift) can help make it difficult for Alistar to land a combo on you, or make for an easier escape after he already has.