Following are some tips on how to keep the four species, the arba minim in Hebrew and which include the etrog (citron), the palm branch and myrtle and willow twigs and which are held together and waved during the Sukkot holiday, except on the Sabbath. The holiday is seven days in Israel and eight days outside of the Holy Land.
The etrog is the easiest because it is a fruit and does not need to be refrigerated. It should not remain in the sun and must be protected from damage when not in use in order to prevent a bruise from making it unfit for use during the Sukkot holiday. It may be possible to preserve the etrog until next year by keeping it refrigerated in a plastic bag that has no air it.
The palm branch (lulav in Hebrew) should be kept in a plastic wrapping and in a cool and dry place. Some people wrap it in a damp cloth. Never place it in water because doing so will separate the leaves, making it unfit for use.
The myrtle twigs also should be kept in a plastic wrapping and in a cool and dry place, such as inside the door of the refrigerator, but not in a colder area.
The willow branches wither quickly and usually have to be replaced at least once or twice during the holiday. Keeping them in water can "bring them back to life" sometimes.