Keep the leaf in room temperature in a dry and airy place away from the sun. Under a shelf for example. Never in the fridge because it's moist or closed in a cupbord because there is no air. It can be preserved for around 2 to 3 weeks. Actually the leaf is consuming its own gel to survive. When you cut a piece it will heal by itself.
The removed gel can be kept a week in the fridge or frozen for later use. This is better than adding conservatives to preserve it longer.