After investing in the development of employees' skills (training, ..), how can you make sure they won't use these skills to leave you?

There are2 ways

The one i recommend is

Try o make your work environment very good .

And show them their career path so that they think100 times before they leave you . 

The way i don't recommend 

Before the training make them sign on a document that they should stay2 years after the training or pay for it . So that if they leave before2 years you will not loose your investment in money but you will loose the time of course .

 ..............................I agree with Alex Al Yazouri

As someone else said "What if I didn't train them, and they stayed". Having well trained employees is critical and focus more creating a great culture and environment!

That's always happening unless you get a good use of that training from them and give them a pay increase for their new skills and work performance. 

Wish for him the same as you do for yourself.

CAREER path, MOTIVATION on daily basis, MONEY a good package. 

 But in my opinion if he was that good and highly skilled it will be very hard to keep him in a small limited firm he will seek to find better opportunity as you did at some point.

                                                                              Thank you.

I agree with Alex Al Yazouri Senior Project Manager

My point of view is (there is no guaranteed way to make sure that you your employees will not leave you after training.) but as professional brothers here recommended ..... you could offer salary increment for those employees  who make use of the new learned skills and/or make an attractive working environment which mainly include career growth. The last way is to make your contract for a certain period like3 years or to add conditions for new joining employees contacts not to work for competitors2 years after leaving your company

there are many reasons for employees to leave a company try to think more in improving the work environment in your company than being afraid of improve their skills as well as you should have a good working contract keeping your and their rights

There is nothing to do to stop them from using these skills to leave ..... because all training that use to develop employees skills have the same procedures and mostly are the same in most company's,, and if there is any different in the training it will be in the environment  ( the place where they been train to work in )but again in general most training procedures have the same roles ,,....

Give a goal to acheive with a good reword and show that their will be upgrading to the next level in due time

The benefit of training and development are improved efficiency and less supervisory burdens; a part of that benefits shall be shared among the employees to recognize them and make use of it to feel them more responsible in their positions occupied.