This is new to me...hopefully it hasn't been posted before:
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Southwest Airlines is allowing customers to check in and print boarding passes 24 hours before flight time instead of its previous practice of midnight before the departure. The new process is thought to be more customer-friendly.
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Requirements for Online Checkin

To use Online Checkin @ southwest.com and print your Boarding Pass, you must:

o Check in beginning 24 hours prior to your scheduled departure
o Have a Ticketless reservation
o Be checking in for flights that have not already departed
o Have purchased an Adult fare type (Customers holding reservations for group, infant, youth, or military fares, and those requiring an additional seat must proceed to the airport for checkin)
o Customers traveling on a Senior fare must be age verified before using online checkin.

To proceed through Security Checkpoint, you will need a government-issued photo ID and either a Boarding Pass or Security Document. Customers under 18 years of age are not required to show government-issued photo ID.

If your reservation does not qualify for Online Checkin, you may still be able to print a Security Document.

Additional information regarding Online Checkin @ southwest.com