Eye Contact
When you sing, you are telling a story to the audience. Look at them as you tell this story and they will respond. Obviously you cannot look at everybody, but you can pick out certain people (e.g., someone wearing bright clothing or someone with a bald spot on their head).
Another option is to look towards the people in the middle of the audience. Change your focus from time to time to include all sections of the audience.
Everybody will feel you are communicating with them personally and will enjoy your performance more. Remember that when people go to hear a public performance, they are looking forward to having a good time.
This means they are automatically prepared to like you even before they see or hear you, so in reality the performance should be a positive experience for everyone involved.
Another important aspect of any performance is eye contact between the performers. The fact that an ensemble are communicating well and obviously enjoying themselves makes the audience feel good too.
