If immediate evacuation is necessary, ask the person what assistance she/he needs.

Persons with mobility impairments who are able to walk independently may be able to negotiate stairs in an emergency with minor assistance.  Untrained personnel should NOT evacuate individuals with mobility impairments unless the situation is life threatening.  If danger is imminent, the individual should move to a "safe" area and wait for trained personnel.  If someone is in imminent danger and cannot be moved to a "safe" area to wait for assistance, it may be necessary to evacuate him/her using a two-person lock-arm position, or having the person sit in a sturdy chair (preferably with arms); THIS EVACUATION SHOULD ONLY BE UNDERTAKEN BY INDIVIDUALS WHO HAVE HAD RESCUE TRAINING.  Before taking action, always ask the individual their preferred method of assistance.  If there is no immediate danger (detectable smoke, fire, or unusual odor), and no reasonable options to evacuate appear feasible, the person with a disability may choose to stay in the building using the other options until emergency personnel arrive to complete the evacuation.

Someone using a crutch or a cane might be able to negotiate stairs independently.  One hand is used to grasp the handrail; the other hand is used for the crutch or cane.  In this situation, it is best NOT to interfere with the person's movement.  To assist, offer to carry the extra crutch.  Also, if the sairs are crowded, act as a buffer and "run interference."  Mobility aids (i.e., canes, crutches, walkers, etc.) should NOT be left behind if at all possible, and should be returned to the owner immediately upon reaching safety.   

Source:  ASD Project, Utah State University, Version 4.0, copyright 2003-2006.