There is no secret formula other than work, work and work. well,.. there are some tips that have worked for me!
* Make the students LOVE Aikido.
* Have good Costumer service, talk to them after class and find what is the reason for them to be in the dojo,.. and give them some of that.
* Keep your training and don let them get bored, teach some of this,.. some of that,.. show them how have them improved their skill.
* The first Aikido days are the more difficult, I remember that when I started aikido, I had a lot of pain in my shoulders and back,... and some times thought two times before going again to class,.. after a month pain was gone,... explain that to your new students. I think it is normal.
I also think Aikido is not for every human on the earth, if someone after some Aikido training doesn't feel good and wants another kind of MA,... well,.. there are some other people that will come later. It is sad to see a student go and never come back,... but,.. that is part of your own learning experience,....