I have used 'liner repair glue' (about $9 from pool shops) with good, long term results. An easy way to stop the sides of the bladder sticking together is to cut a small (20mm) square of baking paper, roll it up and put it inside the bladder before you glue the nipple back on. Position the baking paper under the air hole before you glue. I have not bothered to take the paper out (just left it floating around inside the bladder). If you need to glue any punctures at a later date the paper can be repositioned under the puncture before glueing.
Hope this helps

In my experiance, the main cause of nipples coming unglued is heat. I once left some kites (uninflated)in a car on a hot day and from two kites there were 5 nipples that came unglued.