Easily add startup tasks to OS X/macOS using launchd services. Go4Launch does all the behind-the-scenes work and creates a “template” shell script that you can modify (e.g., sudo vi /Library/BootScripts/go4launch.sh). Add your own commands to the shell script, save, and you’re done.
Download a pre-packaged DMG of Go4Launch at https://13cubed.com/downloads/go4launch.dmg.