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.