The Postal Inspection Service welcomes consumer complaints about fraudulent diet products, but doesn't receive many, according to postal inspector Don Davis. Maybe "they didn't want to admit they got snookered into something useless," Davis guessed.

Here's where to complain if you think you've gotten snookered:

U.S. Postal Inspection Service. If you've ordered and received a product that you feel is a fraud, send a copy of the ad (if possible), plus a copy of your canceled check to notify the service as to where the company is banking. If you do not have a copy of the ad, include where you saw the ad and information about what the product claimed to do. Include your name, address and phone number.

Or, if you simply read an ad you believe to be fraudulent, cut it out and mail it to the postal inspectors. It is not necessary to include your name and address. In either case, write: Chief Postal Inspector, 475 L'Enfant Plaza West SW, Washington D.C. 20260. Attention: Fraud Branch.

Food and Drug Administration Office of Consumer Inquiries. This office can tell you whether the FDA knows anything about the product in question or has taken action on it. If you have a specific complaint, staffers can steer you towards the appropriate FDA office. The Office of Consumer Inquiries also has information and free brochures about body wraps, diet pills, electric muscle stimulators and how to avoid health fraud. Call or write: Office of Consumer Inquiries, 5600 Fishers Lane, Rockville, Md. 20857. 443-3170.

Better Business Bureaus. Will give out any information on customer experiences with a particular company, which includes complaints. They do not conduct legal action, although they do report past legal actions to consumers. To find out which bureau has information about the company you wish to inquire about, call the Council of Better Business Bureaus in Arlington. 276-0100.

The Council also publishes a very useful brochure called "Tips on Weight-Loss Promotions," which includes information about the ineffectiveness of various diet pill ingredients. It is available by sending 25 cents and a self-addressed stamped envelope to: Council of Better Business Bureaus, Inc., 1515 Wilson Blvd., Arlington, Va. 22209.

Federal Trade Commission. This agency does not take actions based on a single consumer complaint, but looks for trends to identify companies that may be selling fraudulent devices. Send information to: Public Reference Branch, Federal Trade Commission, Washington D.C. 20580.