I have always just gone to the gate with my ID and politely requested a ride as a jumpseater. Have your passport ready in case they request it. Smile and be patient and have plenty of back-ups if possible. It's been very difficult getting to and from mem on delta since air tran has discontinued service and delta has reduced service to mem. The seat map seen on line is usually not an accurate representation of available seats. I have left the house after looking at a seat map of an airplane wide open and then get to the gate and they are asking for volunteers. I have been quite lucky also and received a seat no problem. Best of luck to you.