When my brother and I were playing on LAN for one of my worlds we defeated the ender dragon and wanted to refight it. I looked at a few online helps but they only tell me how to do it on a server.
In the Snapshot 15w44a, you can re-summon the enderdragon by placing end-crystals on the edges of the portal in the end. This will maybe change (its just a snapshot version), but this should be the intended way of fighting against the enderdragon again and again in survival without any commands/cheats. See the changelog here.