Check and confirm that there is indeed an error on your bill on the doctor's part by ensuring it is not a mistake of yours or your insurance.
Call and request an itemized bill from your doctor showing the various services rendered, their dates and prices. This helps identify the errors incurred during the billing process.
Counter-check that each service stated was rendered to you and none are duplicated, omitted or added. For example take note of medication that was never prescribed or tests that were not done.
State your findings and reasons for disputing the bill and ensure you mention dates and attendants that served you, such as the pharmacist, if you remember. This helps serve as your proof in supporting your case.
Address the letter to the doctor in question and ensure the mail is certified preferably via notary.