Be sure to change /Users/luomat/ to whatever your $HOME directory is. AppleScript from http://apple.stackexchange.com/a/71755/9226

sync.com.tjluoma.cleandesktop.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>sync.com.tjluoma.cleandesktop</string>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/bin/osascript</string>
		<string>-e</string>
		<string>tell application "Finder" to clean up window of desktop by name</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
	<key>WatchPaths</key>
	<array>
		<string>/Users/luomat/Desktop</string>
	</array>
</dict>
</plist>