If Plex crashes upon upstart, try the following:
sudo mkdir /var/lib/plexmediaserver sudo chown -R plex.nogroup /var/lib/plexmediaserver
This was happening to me after a fresh wipe and install of Plex. When running tail /var/log/kern.log, I get several "init: plexmediaserver main process (5424) terminated with status 134", with various PIDs, but always status 134, and then finally "init: plexmediaserver respawning too fast, stopped".
The above fix is for an older version of Plex, but seemed to work in my case. My setup is Ubuntu 12.04 LTS, x64, Plex ver 0.9.6.3.143 from the repositories. It seems the Ubuntu packager, Tobias Hieta, has made this mistake in the past because I found this in the Plex forums for Ubuntu 11.10.