I find it to depend on the person.
Some people actually gets easier, it's not an empirical tree structure but rather an intuitive feel: You're collecting lots on instinctive data points about someone, but it's less about the points themselves and more about the relationship between the points, what is larger, what is smaller, the context, and other things.
I don't know. Either I am full of it, or you just learn to develop a sense of those patterns of traits.
Example: This guy I saw once recently? I met him on MySpace. When I first met him, just based on his profile and the sort of things he put there, I was figuring him an ENFP or something. (That was based on limited data points, and the obvious ones -- the cliches.)
But within about 15-20 minutes of talking to him on the phone (maybe less, I'm not sure), despite his high degree of openness in conversation, I was guessing he was ISTJ instead. That feeling only got stronger the longer we talked.
Sure enough, when I went to his house for dinner, it was just SO bad -- the cliche ISTJ. Clinched it beyond belief, as soon as I saw him, his yard, and his house, and the things in it, and how he behaved all night. It was crazy... one of the most Si+Te people I have ever met.
So beware the initial data points and using them as isolated reference points to pinpoint type. It's the relationship between different types of points that paints the picture.