Pick the right moment
Too early and your client forgets again. Too late and there is no time to reschedule. You set how many hours ahead yourself.
Keep it short
Date, time, treatment and with whom. Your client needs nothing more.
Pick the channel
By email or WhatsApp, or both. WhatsApp is usually read faster.
Also when cancelling
If you cancel an appointment, your client can automatically get a message. Per cancellation you choose whether that is needed.
Respect your client's choice
For each client you see whether they want reminders and news.