Once your done List becomes long, you should consider unused cards.
To archive a Card, simply select the Archive button in a Card.
You can do the same using Quick Actions in the Board's view. Hover over a card, then select the pencil icon on the left top side of a card. Now go to three dots (more) button and select Archive.
Once you enter the Archive, you will see all cards that have been archived in this Board. You can look for a special card using the Find field.
From this place, you have the possibility to send archived cards back to the Board. Click on Send to board button to achieve that.
You can clearly recognize cards in MyBoard or Search section that have been archived in the original Boards.