Booting into single user mode by default
Open Terminal.app and enter:

sudo nvram boot-args="-s"


In the future your Mac will always boot to single user mode until you remove (sudo nvram -d boot-args or sudo nvram boot-args="") or replace the boot-args.

Other boot-args directly modifying the boot mode:

sudo nvram boot-args="-v": verbose
sudo nvram boot-args="-x": safe mode