In addition to Shimanglo's suggestions, you can get chain catchers, K-edge, Rotor, Deda & Token all make them, which will stop the chain from falling off the inner chain ring, by physically stopping it.

You haven't given any detail about the spec of your bike, but you can look at the Tech Docs from SRAM or Shimano for the relevant components to adjust yourself, in addition to the Park guide linked to above.

For the distance you have got out of your chain, 12000 miles is alot, you may want to invest in a chain checker, and replace the chain more regularly as it needs it, before it snaps in future.