There are a lot of things that are hard to communicate in ads because they are things that can't really be measured. You might want to look at using customers in your ads to have them talk for you about why other companies should trust you. Testimonials are a useful, so long as they are genuine. The other thing is to look for ways you can concretely demonstrate your trustworthiness. How do you prove it? If you can demonstrate that you have a solide product/service, it will be more powerful than saying "Trust us on this one, you can trust us."