So, funny thing: a person's body language may or may not tell you something about what they're feeling, but it absolutely has an impact on how you perceive them. When you get advice to make eye contact, or not cross you arms, or ball your hands into fists and cock your arm (no, dude, I wasn't being aggressive; body language theory is such bullshit), it's saying less about what your internal mental state and more about how you're non-verbally communicating to others.
The bullshit part about body language isn't that it's not valid, because it is. The bullshit part is that there's some key that let's you interpret it unambiguously.
Some people avert their eyes when they lie. Some when they're disinterested. Some because they have social anxiety. I do it because I simply can't think clearly when I'm staring into someone's eyes, because I'm too busy drinking their souls. But when someone is talking and you avoid eye contact, whatever your reasons, they will tend to feel as if you aren't interested.
Anecdote time! The aunt of a friend was a local politician when Bill Clinton was running for re-election, and he stopped in town, gave a speech and mingled, so she got to meet him. She said his most amazing characteristic was that, when you were taking to him, you felt as if you were the only person in the room. He had no distractions, his eyes didn't wander to more important people, he wasn't thinking about other, more important things: when he talked to you, he had all of his attention focused on you, and was only listening to and talking to you.
That's what I think of when I think of effective body language. Regardless of what really was going on in Clinton's head, when he talked to someone, he was able to make them feel as if that's all he was doing: listening to and talking to them.