Before reading other responses:
Since my children were old enough to reason, I've made sure that they're involved in their own care. That means that when we're at the doctor for an illness or even a well child visit, and the doctor turns to us and says, "Why are we here today?" I'll give a brief rundown of why we're there. Then I turn to my child and say, "Honey, why don't you tell the doctor what you've been feeling and describe where you're feeling ____." If any additional details are required, I add them.
That approach has taught my children to self-advocate for themselves with their health, so it extended to when my eldest (12 yrs old) was being evaluated for ADHD. She actually started the process herself and requested the evaluation from the school psychologist. She came home and nervously asked me to take her to her little sister's neurologist for an official diagnosis and to give permission for the school to evaluate her.
After a series of sessions with our neurologist and a therapist, after some drawn out and agonizing discussion, we did come to an agreement with our daughter that a medication would be best for her as a tool to help her. She knew it wouldn't be a cure and it would still take a lot of work, but it would help her a lot.
Now, this is a child who HATES HATES HATES taking any sort of medication. She vomits up liquid meds. She can tolerate pills if they're not too big and she doesn't think about it while taking it. I make sure she knows she MUST take this medication, but I let her be the one to put it in her mouth and drink it down with what she chooses. She's 12 so she's earned it. She does wonderfully and has proven she can do it. Sometimes she gags, but she makes great effort and knows she needs it. She hates how she feels when she doesn't take it.