First, some basic instruction on whom to contact and how to do it can be found here. At the bottom of this page is an example for getting started on your letter. I also recommend this paper as a good summary of the law as it stands and the issues at stake in changing it.
Here are some suggestions for points to cover in your letter (courtesy of opposition volunteers):
• Open with your statement of your position: Example - that you oppose the McCain bill (or any similar legislation) to extend NAGPRA's definition of Native American to include all people on the continental US before European contact.
• The actual intent of this legislation is not clear. The impact
raises legitimate political, social, and scientific concerns:
• The First Amendment prohibits using any religion as the basis for
public policy.
• Modern American Indians would have the singular privilege of claiming remains or other items to which they have no relationship.
• Prehistory is complex and our understanding of the past should not be limited to one explanation.
• Identity of human remains should be established not assumed, then imposed.
• The peopling of the Americas is of interest to people world wide. The ancestors of American Indians did not evolve on this continent, they came from elsewhere.
• The Ninth Circuit Court of Appeals found that the government's claim that 'or was' is implied in NAGPRA yields an absurd result. Making it law will yield an equally absurd result.
• This and future generations (including those of American Indians) should have access to a factual information about the past.
• NAGPRA's requirement to establish cultural affiliation for a claim
does not protect ancient remains if it is ignored.
• An example: The Army Corps of Engineers assumed Kennewick Man was a Native American and assumed that the Umatilla et al. claim was legitimate. A lawsuit was needed to stop the Corps from simply handing over the remains. The court ordered the Corps to re-evaluate their process and follow NAGPRA's requirements. The result: the claim was based on false assumptions and untrue assertions. The Indian Claims Commission had not established aboriginal lands where K-man was found as the Umatillas asserted. No credible evidence demonstrated any cultural link to the tribes.
• Government's defense in the Kennewick Man case of its insupportable assumptions about prehistory have already set the taxpayer back approximately $6,000,000.
I'll add:
• The panel for the hearing appears to be stacked in favor of proponents of the amendment, suggesting to a concerned citizen that the hearing is for special interests, not the public interest.
(More Kennewick/NAGPRA.)