When you have the time for it, write quality engaging little texts for each post along with a related image. Ask you fans some easy stuff to engage interactions like comments, for example ask for opinions to the seen image, let them rate the content 1-10 etc. be a little bit creative about this. If your page get's a lot of fan interactions it automatically gains a better overall reach, because you reach more likely the friends of the fans. Also try to contact other in the same niche fanpages for possible share exchanges. Also as stated by SnakePliskin and in the excellent thread of Lakimo you can always use fb accounts with a lot of friends to call for interactions and population for a fanpage.