From David Gibson's Eureka! newsletter:
Pre-workshop - Send a text message welcoming learners to their workshop.
Post workshop - Send a thank you text message for their input to the workshop.
Post workshop - Send short tips (or shortcuts if it was a systems training) once a week for the next 6 weeks adding or helping embed what was uncovered during their time with you.
Post workshop - Send links or suggestions for additional resources via a text message.