In folk lore and legend there are plenty of methods. I hope that you are just interested in that sort of stuff, and not actually serious. But here's a list anyway.
Drink water from a wolf's paw-print
Drinking from a stream where three or more wolves have drunk from recently
Commit a sacrilege
Hereditary
Curse
Drinking from a cursed stream
Affixing a lycanthropus flower to your clothes
Being born on a New Moon
Being born on Christmas day
Being born on Friday the 13th
Being scratched or clawed by another werewolf
Selling your soul to the devil
Applying a magic salve to one’s body
Black magic and witchcraft
Eating lycanthropous flowers (white or yellow flowers said to be in the Balkans)
Inhaling certain potions
Wearing an enchanted belt made of wolf skin/fur, known as a wolf-belt
Applying a magic salve to one’s body
Simply wearing an animal’s pelt
Placing a real wolf pelt on one’s body and then drinking beer mixed with blood
Eating the brains of a wolf
Spells and rituals
Having sex with a werewolf
Eating an unborn fetus
Wearing a belt made of the skin of a executed criminal
Being cursed by a witch or troll
Drinking from a lycanthropous stream
Sleeping on a Friday night under the full moon with the moon shining directly on your face
Being the seventh son
Being conceived under a new moon
Not going to confession for 10 years
Being gruesomely murdered on a full moon
Being born on a full moon Friday
Tasting human flesh
Being chosen by the Native American wolf spirit