Successfully extended a future flight credit

This question comes up frequently, and the responses are usually all over the place. I have successfully extended credits before, most recently today. Here's what I did:

1. Booked a regular economy flight, non refundable, that used 100% of the credit (this means I wound up with a bigger credit than before, but I had nothing to lose).
2. Waited more than 24 hours (this may not be necessary).
3. Cancelled the flight. I got a new credit with a cancellation date one year out from yesterday.

The original credit was for a domestic flight -- flights originating from outside the US have different rules, so this might not work in those cases.