I use Alistar a lot. Tower diving at 6 is a bold move. You should keep the pressure on his ADC to siphon his mana reserve. Alistar will focus more on peels and heals if you keep good exchanges going.
Also- as a support, I like grabbing
. I don't like being tied up by CC, and if you're going tank- it's more reliable than Exhaust.
An ADC should get
instead of
.. it's a bit more expensive, but worth it early game.
As for the tower dive. If you can't keep the bull from charging, kite, kite, kite. Alistar is fairly slow, which is why he's recommended to grab boots fast. Keep your distance, if you see him coming, don't assume it's a bluff. I get a lot of assists when people stand their ground, and i pluck them from their turret.
If Alistar has Flash- bring it on CD before the engage. We all know the Headbutt+Pulverize combo. But you don't want to see the Flash+Pulverize+Headbutt combo in full effect.
Remember- watch what items your enemy laners are getting- and build accordingly. Watch how the support prefers to support. Alistar can be utility as well as tank- and some people lean one way over the other.
A utility Alistar won't want to turret dive without an escape.
A tank can be harassed.
Just play smart, and be patient.