Every Sukkot any aravot that I start the holiday with tend to get dried up by the end of the holiday, if not to the point of being unfit for service, at least enough to be ugly. What do you do to maximize the amount of time your aravot stay fresh?
I've tried the paper towel/aluminum foil (my father's method) idea, keeping them in the fridge, and keeping them in water. One year I got a whole lot of them and experimented with around seven different methods for each pair, to see at the end of the week which method would be the best.
The winner (and what I've been doing every year since then): wet them just a bit, and put them in an air-tight wrapping like saran wrap or plastic wrap, then keep them in the fridge. A bit involved but it works very well
Plant them near your residence (after "rooting" them in a vase) so next year you can pick new ones as needed.
If you live in a more arid and hot climate, like southern California, put a few drops of water in the lulav bag. When you walk out in the street, the heat of the day causes the water to evaporate, but because the bag is closed, it has nowhere to go. this disperses the water around the whole bag, and keeps the everything moist. Kind of like a mini greenhouse.
As I found out this year, however, this does not work in a more humid environment. It just makes everything soggy and moldy.
I was just referred to "How to keep your aravos fresh throughout sukkos: The definitive guide for frustrated palm-frond wavers", an apparently anonymous document describing a series of nine experiments, carefully testing different strategies for keeping Aravot fresh. Read the whole thing to see all of the strategies tested and all of the results, including photos. Here are some highlights:
Just leaving them alone kept them pretty much OK for a few days, but then, they dried out.
Wrapping in moist paper towel and foil kept them from drying, but ultimately turned them yellow.
Wrapping them along with the lulav caused rotting.
Standing them in an inch of water was effective, but may be Halachically problematic on Yom Tov.
Other than that, best results were achieved by keeping them, by themselves, in an airtight plastic lulav bag. They stayed fresh through the holiday, and only yellowed a little.
I was in a shul hoshana raba one year and after davening a guy showed everyone his aravos and they were fresh like they were just picked. He said all you need to do is put a little water in the bottom of an aravos bag, enough just to cover the bottom of the bag probably. A teaspoon. And place them in the bag in the fridge. I've used this method since and it has always worked!