Log in to chat regularly so members can see someone is available to chat
Point it out to members, if you see someone is online (board index at the bottom), send them a PM and invite them to the chat room.
Set up scheduled sessions. I know a few members who regularly meet to chat every saturday for several hours.
Make sure when people do go to the chat room that you are there to chat to
When members appear, reply as soon as you can saying "hi". if members see you as logged in to the chat but not replying then in future when they see you in there they may close it without trying to get a reply from you.
Following from 5, when you close you finish chatting log out of the chat room (rather than letting it time out after you close the window without ending your chat session) as that can give the impression you are not replying.
Just a few off the top of my head