• Ignoring context: Crossed arms don’t mean much if the room is cold or the chair they’re sitting in doesn’t have armrests. 
• Not looking for clusters: It’s a consistent grouping of actions (sweating, touching the face, and stuttering together) that is really going to tell you something. 
• Not getting a baseline: If someone is always jumpy, jumpiness doesn’t tell you anything. 
• Not being conscious of biases: If you already like or dislike the person, it’s going to affect your judgment. 

Your first impressions are usually pretty accurate. But whether they are wrong or right, first impressions affect us in a big way and we are slow to change them.

You have to be willing to update them quite rapidly. 

• Studies show that if someone seems extroverted, confident, religious or conscientious, they probably are.
• We all pay more attention to pretty people, and so we tend to take the time to evaluate them.
• If you want to know if someone is good at their job, watch them do it for 30-60 seconds. 
• Funny people are smart: Effective humor production acts as an honest indicator of intelligence in humans.

Focus on unconscious behaviors that aren’t easily controlled and convey a clear message:

• Speech mimicry and behavioral mimicry: if a person uses the same words and tone as you, it's usually a sign he/she feels emotionally in sync with you.
• Trust people who are consistently emotionally expressive in their body language.