After opening a file with Preview, the app blocks it (marks it as in use) until you quit Preview. To easily empty the trash without manually switching back to Preview and closing everything, I found finally this (simple) solution. Open Script Editor and create this AppleScript:
tell application "Preview"
quit
end tell
tell application "Finder"
empty trash
end tell
This simple AppleScript does the work, and with the free application Spark, it's only a keypress away from activation.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070307120951914