Don’t seem desperate. Exude confidence and pride. Make sure that the message - implicit as much as explicit - is: ‘We’re doing exciting things. We’re determined to get even better. Come and join us if you think you’re good enough.’
Make a point of observing supply teachers at work and finding out who are the good ones. Many excellent teachers spend some time on supply and might be persuaded back into full-time work if wooed gently.