Go to www.bbb.org and click on the File a Complain, Business and read the instruction. You can file online and an assigned agent will contact you.
BBB acts like a mediator between you and the company, utilizing its resources to help you resolve issues but there is no guarantee. BBB isn't a law enforcement agency and therefore companies do not have to respond to them. However, often times when companies receive letters from an official organization/office like BBB they tend to treat the matter more serious than if the communication is coming from an individual.
I have had a success of claiming my lost with help from BBB recently. They are extremely helpful.
If the company you are having trouble with is a small local business, you might want to try exposing them to local TV or radio stations. I have lived in many towns that has "Call for Action" or some sort, where a news reporter help exposing a wrong doing.
If you lost is under $5000 (this amount might differ depending on your local regulation), you can also file a claim at small claims court. There is usually a small filing fee. If the judgment turns out in your favor, the company will have to pay for your claim as well as the court costs.
Best of luck.