What do you do with the lulav and etrog after the holiday of Sukkot is over?
First off, I know I said no more holiday questions for a while, but since you posted this question on Facebook directly in response to my Sex in the Sukkah  column last week, I thought it was only fair to follow through and answer it. It's a great question, in part because it has traditional, creative and practical answers. After a long month of holidays spinning me in a thousand different directions, I'm greatly assisted by the opportunity to answer this through a neat categorization process.
Traditional: Usually I recommend articles on myjewishlearning.com because of their straightforwardness, but today, I'm recommending an article about the tradition of beating the willows on Hoshanah Rabbah  because it is complex and nuanced and totally fascinating. Basically, on the final day of Sukkot, there's an obscure and, dare I say, totally weird tradition to beat the willow branches that have been part of the lulav. This article speculates that one of the reasons for the tradition is to, "signify the end of the festival and to render its main implement pasul [ritually unfit]." The article goes on to quote the Mishnah: “Immediately [following the beating of the willows] the children loosened the lulavim and ate their etrogim.”
Another article  (thanks, Aish HaTorah!) talks about the idea of "mitzvah recycling," where an object that was once part of a mitzvah can be used for a future mitzvah. One such application of the lulav is to burn it along with the chametz (leavened food) during Passover cleaning. (I've also heard that the lulav can be used in place of a feather for the chametz search, though I can't verify that one.) The etrog can be studded with cloves and used for the smelling of spices during havdalah. The most important thing I got from this article was not to throw away the used ritual items in a dirty or disrespectful place. Some people even bury them in a geniza as you would do with torn prayer books.
Creative: One of the best things about my last post was that it generated a huge discussion on my personal Facebook page, not about sex, surprisingly enough, but actually about this question instead. I let everyone know that their posts implied tacit approval to be quoted, so here goes. Naomi suggests some of the above related to mitzvah recycling and also says, like the Mishnah, that some people "manage to eat the etrog," but that she's "never found it palatable." Aileen plans to try out etrog-infused vodka as well as marmalade. She also notes that etrog was recently a secret ingredient on the Food Network show Chopped, and no one knew what to do with it, regardless of whether or not they found it palatable! Josh then points out that since an etrog isn't sold as food, it's not regulated by the FDA, so eating it is questionably advised. He also says, "Feel free to quote me, but I can't confirm my information," to which I reply, "My advice column loooves unverified information." Then, on that note, Deborah commented that eating an etrog used to be considered an omen for fertility, so in poor communities with only one or two of the fruits, women would fight each other for them. I'll add to the mix that a friend of mine planted etrog seeds in middle school, and her family's living room housed quite a large etrog tree by the time she was in college. I'll also add that quoting this Facebook discussion feels a lot like quoting Talmudic rabbinic discussions.
Practical: You may just have to get rid of it. Do so in a respectful way, as noted above, but don't feel like you need to add to the clutter in your house (if, you know, you're one of those people who has clutter or is otherwise human) by holding onto it until Pesach, or until you can bury it or even until next Shabbat. You've celebrated Sukkot, you've used the lulav and etrog for their intended ritual purpose and now, whether you beat the willows or not, it's ok to move on.
On that note, it's time for me and this column and our entire full-work-week-deprived community to move on beyond the holidays!
L'chaim (said over an imaginary glass of etrog-infused vodka), and be well,