SUSTAIN is the word that will guide you through the match. Always remember that you must give to your ADC the sustain that he needs to spent the maximum time possible on the lane farming.
But, it is not only that! Diferent from Soraka (the support champion that garants more sustain for the team in the role game), Alistar has a good and very useful Crowd Control with his well known combo W+Q: The headbutter pulverazer :P (WTF?)

ON EARLY GAME (Lane Phase)
You must look at four things:
1. The life of your ADC;
When your ADC's life is running low, you may use your ROAR to heal him or the effect of Relic shield to kill a minion and heal your ADC. The second option is very good in the most of the cases, becouse you will earn some money, you will heal your bro and will not spent your MANA nether will put your skill on CD (cooldown), but be mindful that the CD of the relic shield is BIG LARGE EXPENSIVE, so even knowing that this is a very good option, use it wisely (mainly in the first buy, that you only have two charges).

2. The poke that the enemy team do;
ALWAYS be ahead of you companion. That will make the enemy team to have the focus on you, not on your ADC! And we all know what happens when the tank is focused. But pay attention on who is the opponent that you are facing, becouse some supp with CC can be a pain on your *** (like Leona and Annie).
3. The Ganks (ally or enemy);
On enemy ganks, prepare your headbutt to push over the main damage of the attack. Alistar is tank enough to receive a lot of damage and still using his skills to protect the ally, but the ally must know that all you can do when on disvantage is to put him on safetty. Your team will only get the kill if your companion is yet VERY STRONG. So, don't be shy to warn him to just run and to DO NOT CAIT! 3x2 is not the moment to cait.
On ally ganks or assault, it's your time to shine! You must hit the enemy with a perfect combo W+Q, that will garentee a good CC and will put your enemy on a hard situation. You can also use the inverse combo, to trow him far away from his tower or from his safe place: use your pulverize to stun the sh*t of him, then flash through and use your headbutt, the order of the flash will depend of your distance (in the case of you usin an agressive flash, after the pulverize you must simply walk around the target). Tower dives won't be a problem, but you must have your Ultimate (R) on hands before do it, ok?

4. The life of Ally minions;
HAHAHAHAHA this is a funny thing to do, check me out: When you see that there is an ally minion with low health, you can use your headbutt to push the enemy away and make him loose that farm. You may use your pulverize to, that will garantee a stun and probally some poke of your ADC on the piece of evil that lays on the opposite team (WTF?[2]).

Well, there is no mistery on this fase. What you must do is ALWAYS walk with your team, they will need you a LOT. Don't be shy to die in the name of the safety of the team, as an Alistar, you MUST be an unselfish player.
Only start team fights with your hole team and when you got your ulti in hands. ALWAYS go ahead of your team! You MUST be in the center of attentions. ALWAYS try to push their strong damage dealer (mainly the ADC or top) with your headbutt. I don't recomend to use the combo W+Q to gap close and trow everyone in the air, it is nice to have someone far away from you when jumping in the a*ses of the 5 players. Besides, simply walking/aggressive flashin combinated with Q+W it's a nice thing to do.

Well, that's all that you MUST know to rule them all with the Alistar! Other things that you can do with him will come to you with the pratice. Mind this BASIC and you will yet be an Great Alistar!
As: Me (WTF?[3])