You can include a picture of yourself on your ad. This shows people that you are not afraid to backup your product and that you are not hiding your face behind your website :-).

You can put up photos or testimonials, or a list of the people that had bought your product. If they are people who are well known in your niche online market, then wonderful. It gives you more credibility. People think that if successful people used and enjoyed your product, then it would mean that the product is the real deal!

Show people some results of your products. Any test that you had done with your product, list those results out.

Include testimonials of satisfied customers. Publish all the positive results. Survey your current customers if you had a customer list and list out those positive results by asking them for feedback of your product that they had used.

Publish out any reviews or articles that had been written about your product

If you had written an E-book, do not hesitate to tell your visitors about it. It shows people that are an expert in a subject and that greatly boost your credibility.

The website that your ad is on has to be professional, because people will naturally relate the look of your website with the product that you are selling. You sure wouldn't want to buy a web designing software from a lousy looking website, would you? So make your website professional looking for your visitors, and not just that, your website also needs to match what you are offering to your visitors. A business website should not be design full of colors and large font like a website for children.

Use money back guarantee in your ad. This shows how confident you are with your product and it also removes the wall they have in their minds, the risk of losing money on their minds.