I agree, documentation and objective data are extremely helpful for advocacy. I think it is also critical to have:
1. a clear idea of your child's needs;
2. a range of possible solutions that you believe based on your knowledge of your child and whatever research you may have done might be appropriate to meeting your child's educational needs; and
3. as much information as you can get regarding the options the school has offered or might consider agreeing to--or failing that (or in amplification of it), a list of thoughtful, solution-focused questions to clarify what the options might be.
It can be helpful to come prepared (in the sense both of suggesting them and of accepting them) with objective measures that you would be willing to have them use to acquire their own documentation and data. Preferably data that they understand and are accustomed to interpreting. For example, programmatic achievement testing (e.g., MAP or state-wide testing), curriculum-based placement or summative testing (e.g., end-of-course, unit testing, chapter tests, standards-based testing).
