One must try to keep the hadasim and aravot fresh, and to that end the longstanding practice is to keep them in water. The Mishna states that it is permissible on Yom Tov to return the three bundled species to a container of water where they had already been kept, and even to add water to the container (Sukka 42a). However, the Sages forbade filling a container with water or changing the water in the original container on Yom Tov, as it is bothersome and resembles the action one would take to fix a kli, as it allows the species to last (SA 654:1).
On Ḥol Ha-mo’ed, some people would change the water in which they hydrated the lulav, hadasim, and aravot to keep them fresh. Others would unbundle the species and place the hadasim in a vase with water and wrap the aravot in a damp towel or place them in water. Another way to keep the species fresh is to place them in a sealed carrying case, but this works only if they have not been out for very long; if they were out for a long time and have already started drying out, it is better to first place them in water to revive them.
Some beautify the mitzva by changing the aravot every day, as the main way to beautify the aravot is to keep them fresh (Rema 654:1). Many are content to make efforts to keep them from drying out.
As we learned above (section 2), there is a mitzva to bundle the lulav with the hadasim and aravot. Therefore, when one introduces new aravot or returns the hadasim from the vase, he should not just jam them into the existing bundle. Rather, he should re-tie the knot, gently reinsert them into the koishelakh, or at least add a new knot, to fulfill the mitzva of bundling (MB 654:5).