What's the best way to get participants for your research? Getting customers to volunteer their time to improve your product is the best way I've found to make a huge impact on product design. You need to become fans of your customers and treat them like your favorite baseball, football, basketball, soccer, or hockey player. We all collected trading cards whether it was sports, Garbage Pail, or Pokemon - we were fanatics about them. Why shouldn't you behave the same way about your customers?
Asking customers to volunteer their time generates new ideas, gathers feedback, and gives participants the opportunity to share their feelings about the products they may use one day. Collect names like you did collecting trading cards. Build a database of volunteers to champion your products whether they like them or not, you need all perspectives. Then when it's time to run a usability test or send a survey, you have a captive audience.
If you would like more information about this technique message me or check out the presentation I did at World IA Day.