Here is a primer from Nolo.com on this very topic:
"The demand letter is the centerpiece of the insurance claim negotiation process. In it, you set out to the insurance company your strongest arguments concerning:
- what your injuries were and are
- why the other person is legally responsible for your injuries
- what your medical treatment was and how much it cost
- what your income loss was
- what other damages you suffered.
Your letter should conclude with a demand on the insurance company for a lump sum to settle your entire claim."