This year, as always, my aravos are almost black (today being 3rd day Chol Hamoed).
One year, I found the perfect system to preserve them, but I can't do it any more. For about a year, Ziploc (S.C. Johnson, a family company) sold something called "vegetable bags", made of clear plastic with rows of tiny holes in it, meant to release the moisture of vegetables at a natural rate, so they won't rot so quickly as in a sealed bag. I made a 3-layer wrap: tinfoil on the outside, a damp (NOT wet) paper towel next, then a layer of vegetable bag (cut in half, so it makes a long narrow piece). Wrap all of this around the aravos/hadassim in or out of their basket, and store in the fridge. Rewet the paper every day. I had reasonably green aravos until Hoshana Rabba. If I put the paper towel in contact with the leaves, the leaves turn black. If I do nothing, the leaves turn black.
But this vegetable bag is not sold in the US any more, and I can't even order it from the company (I asked). It is, however, sold in Canada. So I guess, by next summer, I'll have to ask a Canadian contact to send me a box of gallon-size vegetable bags.