If you have developed and implemented the strategies suggested so far, you should have a number of clients stored on your database and will have successfully treated them. Have you any thoughts on what to do from here? Would you wait for them to contact you for another treatment or are you going to keep in contact with them?
If you wish to retain them as clients why not keep in contact with them, offer them further appointments and keep them updated with any new treatments that you offer? Your clients know you and the results you achieve so far, so it should be easy to sell them further services. If you keep in contact with them and they do not get back in touch should you assume that you have lost them as a customer? If so, what do you envisage your next steps should be?
The best thing you can do in this instance is write to them personally, thanking them for their custom and apologise for any action you have done which may have lost them as a customer following your training in cosmetic courses. You can always add an incentive for them to return to you, maybe enclose a discount voucher that can be redeemed off their next treatment or something else that is seen as valuable. There is no real reason to lose clients forever, but you must develop a strategy to regain any temporary loss of clients that you experience.