If you don't see a change in the textfields, do some basic debugging.
1. NSLog the button-index received by alertView:clickedButtonAtIndex:.
2. Confirm that button-index 0 is the correct value for clearing the fields, and isn't the button-index for Cancel.
Instead of assuming your code works, assume it doesn't work. Then add statements like NSLog() or assertions that can show you whether it's doing what it should.