Deleting plist file does not reset app on macOS 10.9+
On 10.9, the system is doing some more robust "caching" of preferences. After deleting the plist file, I fired up Activity Monitor and force-killed the "cfprefsd" process. Be careful: there are multiple processes with this name running and you only want to kill the one running under your own user; do not kill the one running as root.
Doing this seems to flush the preferences cache and on the next run of my app, I get a pristine start-from-scratch launch.
Edit: As reported below, using defaults delete [your bundle identifier] at the command line also appears to eliminate the caching issue. I've had mixed success with this.
I found out that killing the user process cfprefsd will reflush the cache, so your changes will be kept
killall -u $USER cfprefsd