Originally Posted by Spiff
In my experience, either a boarding pass or a copy of an itinerary will suffice.
AS did not issue my connecting AA BP at SEA and when I arrived LAX and wanted to take the AE shuttle an agent wanted proof of a flight out of AA's T4- my baggage claim sufficed as it had "AA 32", my connecting flight number. If I do another AA/AS trip I will definitely be sure to have a copy of my ticket receipt- it would have been possible for AS to print out and itinerary in the gate area of T3 had I not had a baggage claim stub.
Definitely take the shuttle to the AE terminal and change there to the shuttle for your connecting terminal, ESPECIALLY if going T3 to T4 during the day when T4 security lines are long. Using the shuttle keeps you inside security so you won't have to reclear and drops you closer to the majority of AA's gates then entering at security.