If you live in the Atlanta area or have no problem traveling to the Atlanta area, there is a zombie casting call for season 2 of The Walking Dead:

"Extra Casting Atlanta is starting casting for some featured zombie types — looking for men and women 18 – 60 — VERY slim builds ONLY some acting or movement experience is great – They will be posting for other roles soon but for now only very slim types and slim facial features for very specific roles, must be available to work on southside of Atlanta — email clear recent face shot and body shot to: twdextras@gmail.com "

If you caught any of the season 1 DVD extras, you'll know that they put all zombie extras through some "zombie school" training classes to turn you into the perfect zombie.