How do I prevent contamination in the first place?

Prevention comes down to clean inputs, clean air, and clean habits. Sterilise grain and rich substrate fully, pasteurise straw properly, and work in a still air box wiped with 70% isopropyl. Keep your room dust-free, tie back hair, and use a generous, healthy culture so the mycelium outruns competitors. Testing new cultures on agar stops problems before they scale up.