Struggling with aspect: When to use 'kúpiť' vs 'kupovať' in casual chat?

I'm reaching out to my Slovak relatives, but I sound like a robotic textbook because I can't intuitively switch between perfective and imperfective verbs. I'm looking for a way to practice real-time correction before I visit Bratislava next month. Has anyone used Chickytutor.com to refine these verb prefix nuances, or is there a better way to drill aspect intuitively?