First, I would make sure that the customer I'm on call/in meeting with understands from the conversation that you are there to listen and get their feedback. I've done a few of these calls and sometimes, because there could be many that I do, I fail to mention this at the beginning. Many customers are not familiar with the Lean Start-Up methodology and so they assume that you are trying to push a product on them. Many times they are pleasantly shocked and offer much more input when they know that you are open to their problems not just aiming to sell your product.
Offer the customers the product for a discount. I would say "for free" if I knew more about what you offer since sometimes it is viable to offer it for free. But, sometimes the sheer price of the service/product is just not feasable to do so. However, the benefit of feedback from these early customers is so huge that in many cases it would be of much bigger value to give the product away for free for a year.
What I have requested in the past is a few things:
1. a meeting where i can watch a potential customer using my product without any feedback from me
2. ask what they think the benefits of the products are (this can serve as very important marketing copy for you because it comes straight from the customer themselves)
3. ask how often they would use the product
4. how much money and time it would save them
5. if the service/product requires a bigger team or more time for the user to try it out, ask them to try it out for a week or two and have weekly/daily/monthly (whichever best applies) meetings with them for feedback purposes
6. ask if they would pay for such a product and, if not, ask them why and what type of features it would need to have in order to get to a paying product.
I'm just going off of what I have done in the past and hopefully this helps you, but, in reality I would think about the questions you need answered before your business is successful and figure out how to leverage your relationship with the potential customer to answer those questions best.