Life drawing and observation are the best exercises for maintaining individuality. If an artist is having difficulty expressing individuality in a "style", then the artist isn't practicing enough outside of that style. One notices subtleties and other uniquenesses while completing observational drawing that otherwise go unnoticed.
If you're still having problems displaying unique qualities, I would suggest taking a pencil and sketchpad to a populated, public area, and start drawing what you see and then reapproach the task freshly afterward.
Also, if the restrictions of a style are impeding on the ability to represent such differences, then it's time to question the value of the style.
Case in point:
(GotG v. 2 spoilers ahead)
Funko Pop (the remarkably generic pop culture figures obviously marketed to those that never grew out of mall-core) figures have a style that rely on big heads and large, round eyes. They've released figures depicting the Guardians of the Galaxy character Groot in four iterations based on different ages:
Adult & Sprout,
Baby, and
TeenHowever, the primary distinction of the various ages of Groot character designs (adult, sprout, baby, teen) is specifically in the character's proportions. So, when the Funko Pop figures, which have a static proportion, were created, they basically all looked the same, relying on superficial differences. In a nutshell? The Funko Pop style is too restrictive to produce anything of merit, especially in regards to characters defined by traits that contrast the style guidelines.