As a general rule of thumb, you want to approach early adopter type folks. They're the ones that are willing to invest the time to understand your product and make early concessions to see it come to reality.

Usually I get them to commit at my cost, but pay up front and if I don't deliver, then give them their money back. Also, make it exclusive, that your only signing X amount of these early adopter agreements.

Give them a reason to take action today.

Again, don't forget to qualify if their early adopters / they're the best kind to get involved - sometimes smaller, but way more engaged and will help you learn faster.