Never pay incentives for feedback/testimonials; it kills trust.
If people are not naturally providing feedback, or even when you're explicitly asking for it, you need to work on your WOW factor. Nobody talks about ordinary products or services.
Moreover, you may not be sincere enough in your asking. First, thank them for being a customer and tell them how grateful you are for their business. Be human. Sign off with:
- have a great day Tom
- enjoy your weekend Dave
- hope you and your family have an awesome and relaxing Thanksgiving
- take care Lucy
- basically anything that doesn't sound like an automated response
Make them feel like their feedback is extremely valuable to you and the community. Nobody wants to put in effort if it's for nothing. State that you'd like to feature or publish their comment and associate with it their full name, city, state, URL, etc.
Finally, you must ensure that your emails are being delivered. Maybe nobody is reading your requests because they are in the junk mail. This can be a very technical topic beyond the scope of this forum.
As an alternative to email, you may want to physically mail your request with pre-paid postage or instructions on how to submit a testimonial electronically.