There are good ideas here. Now it's my turn. My credentials? I worked at the Side Door coffeehouse (folk club and house concert combination in New England, USA) for several years. That coffeehouse is still going strong. My position there started as an audience member and evolved to being one of the organizers and planners.

First and foremost for success is consistency. Decide when to be open and always be open then. You will have some night when nobody will show up and other nights when you will have standing room only.

Light the stage and give the audience the anonymity of dim lights.

Have a decent sound system but don't overwhelm the audience.

Welcome the audience and stress how important they are to the success of the venue.

It has been my experience that the best combination of acts is to have an open mike followed by a known act. I have heard Jez Lowe and Gordon Bok both express their happiness at participating in such a mix of performers. The amateurs bring additional audience members to hear only them and a percentage of those will be back in later nights.