Thanks.. good tips so far. The income has been steady for about 2 years now.. so I'm thinking maybe the best thing to do is just to do some seo, focus on that and increase traffic.. that way I don't have to add more ads either.
Still, I think you should show minimal ads to logged in users too. Maybe the adsense ad link format (plain links).
When something is free, users should expect to see some ads.