I agree that it's a good idea to be as open and friendly as possible. If one of you is more diplomatic and less likely to be uptight during the meeting, I'd get your plan of attack straight in advance and let that person do most of the talking.
If you have evidence supporting your requests, then consistently refer to that evidence (test scores or what have you). If your evidence is in the form of work samples from home or past years, you might consider bringing them to the meeting and offer copies. If there is no recent testing or you believe that past testing was inaccurate, consider asking for testing to either be administered by the school or accepted from an outside tester.
Be alert to vagueness. One problem that we had early on was that vague plans were made with no definite deadline, and complete delegation to the teacher, resulting in little or no implementation. Ask politely for specifics on timeframes for any follow-up steps such as planning meetings, deadline for implementation of any accommodations, exactly what will be implemented, etc.
If you want subject acceleration, I would personally think about advocating for a subject pull-out (going to a different class for a subject) and possibly fall back to in-class enrichment from there. You might browse this site, Hoagie's Gifted and A Nation Deceived for info to bring to the meeting or supply before or afterward, in arguing for acceleration. Does your district and school seem open in general to acceleration, or more likely to provide in-class enrichment?
Offer to shoulder any of the burden you can; this plays well even if it's often not accepted. Throw lots of little supportive and appreciative phrases and burblings in at odd moments. But above all, as long as you remain polite, don't be afraid to be a little bit pointed in stating exactly what you want, openly though politely disagreeing when necessary, and asking for details.
If your school district is like ours, the school will generate written meeting minutes. If they're not provided, ask for them, and follow up to correct any inaccuracies as to what was agreed upon. Also, even if some things are agreed to orally as you go forward (for instance in a phone conversation with a teacher), find ways of following up by email to document what was said and done, without being too obvious about it.
