Then, praise the bird profusely and give it back its previous perch. A cockatiel on your shoulder might start climbing down, for example. Don't use a key phrase common to daily language. Try again in a minute or so, though. Also check your shirt or the floor to see if the bird went while you weren't watching.
Some birds have a stronger instinct than others.
But remember all birds are still individuals. In the long run, it usually is up to the instructor's patience and persistence.