* Bones: Small animal bones, polished and used as hairpins.
* Twigs and vines: Twisted into circles or clips, maybe adorned with berries or leaves.
* Feathers: Raven feathers, owl feathers, or other symbolic bird feathers.
* Crystals: Small, polished crystals attached to pins or combs.
* Spells: Maybe they just use a little bit of magic to keep it in place!
* Ordinary objects imbued with magic: A plain wooden hairpin enchanted to always stay put.
The possibilities are endless, depending on the type of witch you're imagining!