Set boundaries.

At my first professional job, there was one guy who always came in at exactly on time and never stayed late. I always thought it was weird, I'd typically stay at least long enough to finish whatever thought I was working on, and sometimes later just because I had nothing important to get to. Eventually I became one of the guys to go to when you needed someone to stay late. I didn't mind, in fact I like being helpful. Looking back, I realized that I gave the company a lot of free work and didn't get anything for it. It seems obvious but important to realize from day one, you are setting expectations. A "good" manager will figure out pretty quickly which employees they can exploit and how.