I tried modifying a preference pane plist (with defaults write), but the change doesn’t take effect. Is there a way to make that happen?
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.
killall cfprefsd && killall Finder.
You could also restart your computer.