This is Aurora Tang's method of sealing up her balloons. I think its the best I've seen. It was developed during a project to document Robert Smithson's Spiral Jetty with the Getty Institute. She has used it extensively for landscape photography.
1) put a cable tie around the balloon neck, holding it in place around the helium nozzle.
2) when balloon is full, pull the balloon neck off nozzle while tightening cable tie.
3) run balloon neck through steel ring and fold back on itself, wrap another cable tie around doubled back balloon neck and tighten.
4) add another cable tie.
These parts are included in the PLOTS balloon kit, only with re-usable cable ties.