We are shown in the original Star Wars trilogy (and even in the prequels...) that Jedi generally require years of focused training to develop mastery of their Force abilities. Notwithstanding certain characters in the most recent film, we can assume that the mind trick is likewise a power that must be developed through training. Since (Qui-Gon), Obi-Wan, and Luke all utilize the mind-trick at least once, we can assume it was a relatively common skill.
Since it would seem to violate the Jedi code of ethics to grab a random person and practice mind-tricks on them, who do Jedi-in-training practice the mind-trick technique on? My first thought was that they would practice on their teacher, but that sounds like trying to learn lightsaber swordsmanship by hitting an AT-AT with a stick.