The best way is to offer early adopters a discounted rate and lock it in for life. Make it a no-brainer for them with the understanding that they are your beta group and that you will ping them from time-to-time for feedback on concept improvement.
This is such a crucial step that must be done right. We are currently navigating this stage on one of our projects. One way to identify the right prescribers is using targeted ads in networks like Facebook or LinkedIn. These networks allow you to be very specific in who you target. Twitter as well. Running a simple campaign on each platform can land you a decent list if prescribers. I'd be happy to chat with you in more detail if you'd like.
I interview successful successful entrepreneurs at treptalks.com and I usually ask entrepreneurs this question. Most of them emphasize #1 below.
Your question is very general without any specifics so the response is general here.
1. Ideally, if you can find your prospective users/customers to get to sit with you in-person, you can get the best feedback. You will have to come from a place of trying to help them solve a problem that they are already having - otherwise it may just seem like a waste of time on their part. In other words, they would be more interested if you are talking about solving their problem and then showing how your product may do that and then asking for feedback.
Can you think of places where you can meet the kinds of people who would potentially use your product?
2. If you have an email list with people who may already be using your product, you may want to create a short survey asking questions and sending them an email with some sort of incentive for completing the survey.
3. Again, if it is web based product, there are services like usertesting.com and I am sure there are others that provide similar services where they actually recruit real people to test out your product and gain feedback.
You may want to check out a couple of interviews below where entrepreneurs discuss the same topic in depth.
Hope this helps. If you have any additional questions feel free to reach out.