Reload modified system plist
Assuming a newer version of OS X (10.9+ ?).

You have to kill two processes: the process that manages the preference you are modifying (e.g. killall Finder if it is com.apple.finder that you are modifying) and "cfprefsd".

"cfprefsd" is a newer preference caching service of OS X.

In Terminal.app: killall cfprefsd &amp;&amp; killall Finder.

You could also restart your computer.