This is the best method. Use whatever boot loader you have (and you can provide your own via CD, USB, PXE, floppy, etc) to add a kernel argument to run a shell as init process. `mount -av` once booted should bring the filesystems in line.

If you find the BIOS is locked, the drive is encrypted, or you can't run your own bootloader (or edit theirs), use a DMA [1][2] or cold boot [3] attack. This should be the first thing you attempt if the machine is already booted up, in case the drive is encrypted. (Also trying a network attack using metasploit before trying the cold boot attack might net results)

  [1] http://www.breaknenter.org/projects/inception/
  [2] https://github.com/mrbreaker/mofo
  [3] http://www.linuxjournal.com/magazine/cold-boot-attack-tools-linux