How can I prevent my allergies from being triggered by sawdust?

Dust mask. I would look into a high quality dust mask or even invest in a respirator if you work regularly with wood. I would also wear goggles or safety glasses to prevent eye problems. You also may need to use allergy medications prior to working in the woodshop.
Dust allergy. Avoidance, wearing a form-fitting and high efficiency particulate mask, allergy shots, salt water washes used throughout the day, etc...