The Many Faces of Alistar [Extensive Guide]:
http://www.leagueoflegends.com/board...ad.php?t=36784
Short and sweet:
0/15/15 or 0/21/9 or 0/9/21 masteries
cleanse+flash/ghost or flash+ghost or flash+teleport
Items to consider = Catalyst early 2 Banshee's veil late. Early Chalice of Harmony, Aegis of the Legion, Frozen Heart, (Soul Shroud if you go mana manip first instead than Chalice), Squeeze Sheen in somewhere if you can. Should rarely get past Frozen Heart in games. If you do Guardian Angel. Some people like to rush Warmog's but your healing should be sufficient with decent mana regen (and Roar)
My personal fav is (thanks XiaoBing):
Chalice of Harmony
Sheen
Swiftness Boots
3x Heart of Gold (then sell 1 heart for Banshee's - sell another for Frozen Heart - etc.)
Runes - Typically Flat HP Quints or Movespeed or Gold or Dodge; Clarity HP or Dodge Yellows; Armor Pen Red; Clairty or Magic Resist Blue