Make Eye Contact
People notice eyes when they look at someone, and they instinctively try to make eye contact with whoever they are looking at or whoever they want to speak to.
If you avoid eye contact or make it impossible for others to make eye contact with you, you will come across as standoffish, anti-social or even shifty.
You need to be careful that you don't stare people down, but you need to look at them long enough to establish a connection and encourage further communication.
As they say, it is all in the eyes, so make eye contact a priority if you want to be considered approachable.