I wrap mine in a wet paper towel, then silver foil, then put it in the fridge.
This. Works every year. Even with the garbage Israeli ones.
How do you fit your whol lulav into the fridge?
IME if Arovos are surrounded by many (36) Haddasim and put back into the plastic Lulav bag, it will keep everything airtight enough so that they stay fresh enough until Hoshana Rabba.
My aravos and hadasim are in a holder. So I slide that off and wrap that up.
Same and I always wonder why this isn't the practice of the masses
See MB 654:5http://beta.hebrewbooks.org/reader/reader.aspx?sfid=14166#p=255&fitMode=fitwidth&hlts=&ocr=
Then the holder gets moldy from the moisture of the papertowel
Keep that part of the paper towel dry
So tie the holder to the lulav with a knot
Can you slip them apart and back together or not?