Another tact you can take with your respondents is to show their input matters. This is about the only actual advantage the research has over an actual product launch.
We find it most valuable to prototype as soon as possible in get it into the respondents hands. We always go through several iterations and most of the time we will go back to a previous site to test a new iteration. It demonstrates we listen and implement what the respondents say. It gives them a great sense of value. Only helps when you use them in the future for a different research.
A launch on the other hand, the customer gets what they get. And while we certainly listen to their input for a gen 2 product, the responsiveness is no where as fast as we do in development.