So what constitutes evidence? Test scores, evaluations, meeting notes, letters, and emails are all evidence. Oral communications are also evidence, but they are often seen as less reliable because they are harder to prove. To deal with this problem I recommend that parents keep a journal that contains the date, location, and content of any verbal communication they have with the school. Record these interactions directly after they happen, that way your memory is fresh.
Another way to permanently capture oral communication is by writing a follow-up letter. If a school administrator promises a service for your child in a meeting, send a follow-up letter to that person the next day and refer specifically to what they said.
Journals and follow-up letters can also be used to document inaction. It's hard to prove a negative, but if you record your repeated requests or unreturned calls in a journal that is evidence too.
Creating a paper trail of your story not only results in evidence you can put forth at IEP meetings, mediations, and due process hearings, it makes everything you say more believable because your statements are grounded in facts.