I think a good zombie starts with a good costume: something tattered and aged, that simply looks nasty. I really don't like when people do zombies and their attire looks clean.
Makeup: Depending on the "age" of the zombie (freshly turned, decaying, etc) the level of rot and decay in the makeup varies. The makeup needs to cover all skin that is exposed (neck, shoulders, arms, legs, etc) for a consistant look.
Acting: I'm a fan of the slower, classic "Night of the Living Dead" zombie, where they lurch and their limbs don't seem to work well together. I like to get intestines from the grocery store and use those as props for the zombies to fight over and "eat."