How do I respond to a 93A demand letter?

No, I cannot post a sample response because, among other reasons, every response will be unique and based on the specific allegations contained in the original 93A demand letter.  However, here are some basic pointers for drafting a response:

Send your response "within thirty days of the mailing or delivery of the demand for relief."  Write clearly and avoid inflammatory or impolite language.  You should assume that whatever you put on paper and send to the plaintiff will wind up as an exhibit at trial.  Because of this, and because of the threat of triple damages, if the amount in question is significant, you should consider getting a consumer law attorney to help you deal with the issue. 

If there is ANY validity at all to the claims outlined in the demand letter, your written response should make a reasonable offer of settlement.  By doing so you limit the plaintiff's potential recovery to the amount of your offer (if a court subsequently finds that your offer was fair) and may avoid the possibility of having to pay the plaintiff's attorney fees.  (See below.)

For the same reasons, even if the plaintiff's demand is frivolous, you should respond to it with a reasonable explanation of why you are not offering any amount in settlement.  Don't simply dismiss the claim with curt or sarcastic language. Rather, demonstrate in your response that you have investigated the claims and found them to be inaccurate or without merit. 

When the response is ready, send it by certified Mail, Return Receipt Requested.  You do not want there to be any doubt in the mind of the judge that you sent a response to the demand letter. 
You should also review Massachusetts General Laws Chapter 93A, especially Section 9, which states in relevant part:  "Any person receiving such a demand for relief who, within thirty days of the mailing or delivery of the demand for relief, makes a written tender of settlement which is rejected by the claimant may, in any subsequent action, file the written tender and an affidavit concerning its rejection and thereby limit any recovery to the relief tendered if the court finds that the relief tendered was reasonable in relation to the injury actually suffered by the petitioner"  Good luck.