No 'sure-fire' way but like everything on a cat it needs to be set-up to work properly in the first place.

-Make sure the main is not binding in the track.
-Make sure when halyard is tensioned the lock ring angles away from the mast.
-Put the boat on it's side with the main up and check the length of the hook. These are often standard components and are often too long. Grind back if necessary.

Note: The cringle position at the head of the sail varies from sail to sail. I have one sail I have to reverse the ring to get it to lock (it does come down easy though).

Cheshirecatman