execute script at startup

What would be the simplest way to launch a script during the boot or desktop startup process in debian? The script in question consists of a single command (ifup wlan0) to connect the wifi and requires root privileges. My system is debian testing, slim login manager, and jwm as window manager. I have tried the following: 1. creating the .service way with systemd. It starts the script, but turns it off at desktop login, can't figure out why. 2. using the startup command in Jwm, but it fails I assume because launching ifup requires root privileges. 3. put the command in the .bashrc file. same as the above 4. place the script in the /etc/rc5.d directory. nothing worked so far. suggestions?