You may be interested in NI's state diagram toolkit. Obviously you can develop state diagrams without it but it becomes very useful for bigger diagrams, all the more if they are subject to change. It is part of the toolkits that ship with the Professional Developer Suites or can be bought separately.
There is also the LabHSM toolkit. Because I have never used it, I can't say more about it but I expect it to be a great tool for complex diagrams.
