Here is what I decided to do to get real feedback on product designs and functionality and to seed my project with customer evangelists who will be early adopters of the product and also (hopefully) spread the word to their peers down the road.
I decided to go for quality over quantity and to really engage a small number of target customers from the very beginning. I wanted to really give them a sense of ownership in the success of our project. So, I created a customer advisory group. I limited the membership to 5 or 6 people, wanting to make it exclusive and special.
I defined the time commitment required by the customer members and the benefits they would receive in exchange. One of the key benefits was to be part of creating something new and valuable in an area that they cared about. That was actually one of the most motivating factors - people want to be part of something creative and new!
I just kicked this off with our first monthly meeting. We are not as far along the dev path as you are. We kicked it off with a dinner and social event followed by a presentation that shared with them the vision, showed them our wireframe designs and then asked them to do a homework assignment. This approach takes a lot of time. I personally recruited each member from my network over a meal or coffee. I believe having a VIP group like this makes a lot of sense for my business, and I believe it will pay off in the end. Some are already talking about others that will be a good addition to this small exclusive group, and one is thinking she may not be in our target market which is also valuable information.
this approach might not work for your situation, but I figured I'd share it in case it sparks any ideas. I love this group for that!