Don't change your guarantee terms every couple of weeks. Ensure that all your products or services consistently meet the quality and value standards that your are promoting. Credibility is something that is earned. Therefore, you need to give of yourself to get trust back in return. There are many ways to do this.
Some software businesses offer free trial periods so the customer can try before they buy.
This process enables the potential customer to check that your product will do the job you say that it will. Sharing your knowledge is a useful way to build your credibility. This can be done through monitoring discussion lists and boards that your potential customers frequent. Answer questions that they pose. Be a regular contributor to the list so that people come to know and respect your knowledge.