Don't you know the look of shame?...well, look of shamING. Think of any teacher you've ever had. He/she would walk into the classroom while all the students were chatting away. Then she/he'd say something and/or give a look to shut everyone up. Cultivate that.

I am only partly being serious. But yeah, eye contact with audience helps to get their attention. If you are "reading" poetry off of paper and your eyes are down, then you don't have much chance. Memorize it; perform it. If music, greet the audience first. Let your intent be known that you are engaging them. Otherwise they will tune you out as "background" music. People are so accustomed to background music these days that when a performance happens they often assume it means "OK everybody, now is your turn to talk freely." The "backround" even makes some people feel more comfortable about talking. as it's a kind of noise that makes their own conversations feel less naked.

I sing at a weekly open mic which is small enough that I actually never use the mic (everyone else does). Funny thing, while other people are strumming guitars and have their head into the mic, I have my head up and I'm in front of mic (closer to audience), even moving towards individuals. I don't know if it's just shame or actual interest in what this weird fellow is doing singing so loudly standing in the "middle" of the room, but everyone shuts up and listens!