1) One is permitted to derive benefit from the four species after the festival. However, one may not use them in a disgraceful manner. It is therefore forbidden to throw them to a place where people tread, and obviously one may not tread on them. The same laws apply to the willow branches used for the beating of the willow on Hoshanah Rabbah. (Latter authorities)

2) Some use the myrtle from the four species as spices for after Shabbos so long as they still have a scent, as things which have been used to fulfil a commandment are fitting to be used for another. (Shulchan Aruch and Mishne Berurah O”Ch 297,4) For this reason also some use the four species as fuel for baking Matzos or burning the Chametz. Others keep the four species and the willows from the beating as a charm for keeping safe. (Latter authorities)