Update 12/1/2015: It appears that the Nexus 7 is now automatically recognised and made available in xubuntu 14.04.1 Trusty; however, if you are still having trouble, read on.
Install the required packages:
sudo apt-get install fuse-utils mtpfs
Create mount point:
sudo mkdir /mnt/N7
Manually mount the device:
sudo mtpfs -o allow_other /mnt/N7
Once finished, manually unmount the device:
sudo umount /mnt/N7
For more info see post on nexus7forum.
N.B. My N7 has a USB ID 18d1:4e41, and on Xubuntu 12.04 it was not necessary to modify udev.