I have flown a few times with a Lulav and like mentioned previously, I would cut the bottom to make it the smallest kosher size (I would ask for a lulav with the shortest shedra, (final leaf split) measure down 32cm (according to R' AC Naeh's shiur of a tefach=8cm) and leave an extra cm or two to be extra safe, then place it in a post tube and keep with me. (I flew MEL-LAX-JFK, needed it in LAX to bentch) I didn't specifically declare it by customs, but did check the box declaring food including fruits and plants, they put my bags, including lulav and esrog through the scanner and said nothing, at the time, I noticed quite a few daled minim confiscated on the table behind me, so I guess others weren't as lucky as me that time round.