Don't get hung up on formal process notations unless you are planning to turn them into software / workflows.
I have done some of my most effective work with pen & paper / flipcharts. For documenting more formally Visio / Smartdraw or other simple diagramming software will do.
Techniques: soft systems for the big picture, use cases to identify processes and actors, activity diagrams with swimlanes for the actual processe.
Agree that somone outside of the process should be lead the documantation as they can ask the dumb questions.
Don't try and document all proceses - use a riak assesment ot identify key processes.
As for keeoping the documentation up to date -how often do the processes change? If they do change as part of a project ensure the project is responsible for updating the documenation
Once models are signed off make sure they are available to those who need them. If the only person who knows which folder they are in goes under a bus you have the same problem!