You can't really practice penalties. Everyone can take good penalties during practice. It's the nerve during the match. It's more on your mental state than anything.
That's not true at all.
What people say is that you can't replicate the pressure of a penalty SHOOTOUT. But Aguero has never suffered from lacking the mentality to be clinical, as 93:20 proves.
But for a run of the mill penalty, you can definitely practice and improve, and add new ones to your repertoire - he seems to only have down to the right in his locker at the moment.