Change SD card to read-only mode.
http://raspberrypi.stackexchange.com/questions/5112/running-on-read-only-sd-card
tps://web.archive.org/web/20150101110424/http://raspberrycenter.de/forum/umruestung-raspberry-pi-read-only-root-filesystem

in:

$ sudo nano /etc/fstab

change:

/dev/mmcblk0p2  /               ext4    defaults,noatime,ro  0       1
tmpfs           /tmp            tmpfs   defaults,size=30M    0       0

disable:
rsyslog, dphys-swapfile