This will depend to some extent on the nature of the service that your firm is providing. However, the most obvious way to prevent your firm's customers from being dissatisfied is to provide them with a high-quality service at a good price. Customer dissatisfaction is often related to the quality of the service and to its price.
Another thing that can be done in all types of service industries is to provide a pleasant atmosphere and to make the customers feel good about the people with whom they are dealing. Customers who feel like they were treated well and who felt that their overall experience was pleasant are less likely to be dissatisfied, even if the service provided is not 100% perfect.
So, the best ways to ensure customer satisfaction are to provide a good service, at a good price, in a friendly environment.