This is required if you plan to go via Spiti valley (optional on the June Run, unavoidable on the September Run).
Route in June - can be picked up en route only in Keylong. Route in September - can be picked up in Shimla in advance or en route in Reckong Peo.
You'll need a few passport photos, your passport, some patience and a few hundred rupees.
It's well worth getting, the Spiti Valley is the bit that really separates the boys from the slightly bigger boys.