The identity planning process begins, and ends, with a detailed checklist.
At the beginning, you will audit all existing corporate communications, to assess the content, consistency and quality of the currently communicated identity message. It helps to have a checklist of the media to be collected.
At the end of planning, the beginning of the design phase, you will need a checklist of the items which need to be redesigned... corporate communications items, and (generally) subsidiary or divisional materials as well.
Here is a one-page checklist I have found useful for 'typical' corporate audits and implementation plans:
PDF: Implementation Checklist
You will want to collect and plan for business unit communications (subsidiaries, principal divisions) as well as strictly 'corporate' communications; add as many columns as necessary.
And of course, you'll need to add media specific to the company and its industry; for an airline, say, add aircraft and ground equipment livery, uniforms, in-flight amenities etc.
For a somewhat longer (two-page) checklist, see BrandLogic's