I practice empathy by roleplaying; it forces me to realize the constraints around the character, regardless of how far apart they are from me, so I can react appropriately to the constructed world.

I also watch a lot of movies and read lots of books, taking 'characterizations' from the people presented there. I imagine their faces, the movements they make, the words they say, when put into particular scenarios. This requires great attention to detail and good pattern-matching skills.

When creating a proper roleplay response, I always have a picture in my head of what's going on, and why, and what the person is feeling. I make the feeling my own, which, at the height of the role, allows the character to surprise even me.