Sprinkle some water on the leaves first. Then take the entire holder with the hadasim and aravos in it off of the lulav, put it in the plastic bag that the lulav comes in, and put that in the fridge.
Or just buy new aravos every day or 2. Considering how much people pay for their esrog, to spend a few extra dollars to buy extra aravos shouldn’t be such an expense. Or grow your own as mentioned above (or find a neighbor who has them like popa)