Getting customer to prepay is difficult if a) you don't give them a reason, b) you ask the wrong type of customer.
** Assuming you're doing this for customer development, not just to build revenues ***
Giving them a reason: I usually say "we have an early adopter program, and we're only signing up 10 customers that will get a discounted rate, and help us co-create the product over the next 2-3 months".
Asking the wrong type of customer: You need to find customers that are early adopters. So what do they look like? We'll, they use technology and or have tried to solve this problem themselves. Maybe it's because they have an Instagram account, or know about SnapChat`? Maybe they donate to Charity:Water, etc. Try and think of things they might do to demonstrate that their early adopters, and approach those customer - not the laggards.
Hope that helps.