Do not carry your bags: lockers and luggage delivery

If your concert is on the day you check out, there is no reason to drag a suitcase around. Several places will hold it.
Subway and train stations have coin lockers. You pay with T-money or a card, and the price depends on the size. Past the time limit, bags are moved to the station office.
At busy stations — Hongdae, Myeongdong, Gangnam — every locker is taken by the afternoon. Drop yours in the morning, or ride one stop to a quieter station.
Hotels and guesthouses will hold luggage both before check-in and after check-out. It is usually free and you do not need to arrange it in advance.
Luggage delivery services run between the airport and city hotels. Use one on your last day if you would rather reach the airport empty-handed.
Some venues will not let a large bag inside. Knowing where the nearest locker is saves you a scramble at the door.