Here's a list of things I used to do when I played alistar in S2
1) never smart cast your W because you will frequently mess up your WQ.
2) Always hold down q after w as this will activate the proper combination 100% of the time. Tapping Q over and over again isn't reliable.
3) Use W on a minion and Q if you cannot w to an enemy directly.
4) Never use your utlimate during the beginning of fights. People just ignore you because they know they cannot damage you.
For S2 Masteries I'd recommend a 4/5/21 page, making sure to grab the CDR in both the utility tree and the offensive tree.
This helps a lot as it gives you 10% CDR at the beginning of the game. If you get really confident with your alistar play, then invest in flat CDR quints and blues. Combined with your masteries this will give you 20% Flat CDR at level 1. From games, I'd say CDR is the #1 stat you need on alistar. After that get resistances. Worry about HP last... He gets over 100 HP per level, so as long as you're smart about how you play and you're smart about using your ultimate, HP should never become a problem.
The one major drawback of using my type of build is that you are far less tanky throughout the game (when your ultimate is down). Because of that you have to become much more selective about your fights. However, my build allowed me to go from 900 to 1400 elo in one season.
However, Ive stopped playing alistar... The main proble with supporting him is when your laner is passive. And if you're up against a poke lane with a support like Janna/lulu/zyra/zil.etc, then alistar becomes a lot harder to play.....
Oh, and as a side note, in team fights you are a good initiator, but you're also a good peeler. If you notice the enemy team is diving on your carries, then never initiate with alistar. Instead, always save his CC to protect your carries. I mean, there's no point in initiating on the enemy if they just ignore you and kill your carries.