Sometimes, when attempting to empty the trash in OS X, you will get an error

"This operation could not be completed because you do not have sufficient privileges for some of the items."

The best method I've found yet for getting around this is to open the terminal window, type in

sudo rm -drf

and then drag in the items from the trash.

or to delete entire directories within the trash:
sudo rm -r
and then drag in the directory that is within the trash folder. BE CAREFUL not to drag in a higher folder, such as the trash folder itself.

Thanks to Pink for the tip.

Other tips on emptying reluctant trash are listed here:
OSX FAQ
AppleInsider