Disable resume for all apps

To disable resume for all applications permanently, in the Finder, press Shift-Command-G to "Go to the folder" ~/Library/Saved Application State/, then press Command-I to Get file info, and check the "locked" box to lock the folder. This will keep any newly installed applications from resuming.

Uncheck Restore windows when quitting and re-opening apps under
System Preferences -> General.

In Mountain Lion, this option has been changed to read, Close windows when quitting an application with fine print to explain that open documents will not be restored.

Or use the Resuminator app:

http://www.tuaw.com/2011/07/26/dear-aunt-tuaw-help-me-fine-tune-session-window-restores/

Allows both fine tuning or global toggle of Resume.