From your experience what is the most important advice you'd give to a new Online Community Manager?

The most important advice I would give to a new online community manager would be to understand the need and interest of the community through feedback session and direct observation on the field and talk to people to understand their requirement before planning any intervention.  Should be a good listener to hear their grievance, all three P ( patience, persistence and perseverance should be a common factor in any community manager) have love which is common towards all the living things be it human, animal or plants. 

Always try to share with your audience real life experience of somebody who has passed with a similar experience as you want them to experience. 

The most important advice I would give to a new online community manager would be to understand the need and interest of the community before planning any intervention plan. a pilot or base line study would be a key advice to understand as to what the community's expectations are.