Travelocity's FareWatcher PlusSM has been doing a terrific job of notifying me of price changes for airfares which are actually AVAILABLE.
Sometimes other companies will send an alert telling you that there is an fare available for say $325. You go to book the flight and the seats at that price are gone.
When signing up for the alerts you specify
Email me when air fares:
Go down by (USD) $25
Go up or down by (USD) $25
Goes below (USD)$: _____________