Bonuses and increased compensation if possible. If not, then increased autonomy, respect, and trust at the very least. If someone goes above and beyond outside of their job description, they're demonstrating initiative and judgement.
What you shouldn't do that a lot of places do is just give the person more work with nothing else. "Good job, now keep doing that AND all your other work for the same salary. That's cool, right? Your boss got a fat bonus for your work, though."
What you shouldn't do that a lot of places do is just give the person more work with nothing else. "Good job, now keep doing that AND all your other work for the same salary. That's cool, right? Your boss got a fat bonus for your work, though."