Think about your natural body space, then imagine it is twice as great, then imagine it fills the whole room. Hold that thought and your body language will automatically increase your presence.
Talk at a measured pace - gushing suggests gullible; steady suggests wise. Don't raise your voice. Stop talking when you've made your point, even if there is silence. Don't worry if it takes others time to understand you.
Look slightly above the people you are talking to, as if the point you are making is more important than their reaction.
Change the role you are playing if you're having trouble gaining gravitas. If the other people know more than you, provide incisive summaries or ask challenging questions rather than compete in an area where you can't shine.
Set the mood - be the source of enthusiasm if the others are sombre, or the voice of calm consideration if the rest are ebullient.
Don't talk over other people or, if you have to, wait until there is silence before you make any serious points.
Give your undivided attention when others are speaking - at least, to start with.
Focus on quality rather than quantity. People with presence say relatively little, but what they say counts.
Take a few notes. It suggests you are sifting the gems or having brilliant thoughts (high presence). But furious scribbling will make others see you as the note-taker.
Don't expect to be liked - gravitas is for gaining respect; if it's love you want, try building rapport, which requires the opposite of many of these techniques.