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.
Your question is all about trust. Do the customers trust your company? But of course, you cannot establish trust overnight.
Secondarily then, this becomes about copyrighting 101.
Is the offer crystal clear, have you provided social proof (testimonials from happy customers), and is your 100% money back guarantee clearly stated on the offer page?
I agree with providing testimonials and refund guarantees to reduce the client's risk.
Some other tactics:
1) Include discounts for prepayment as well - making it more expensive to pay at the door.
2) Include registration deposit (instead of full prepay) .
3) Allow part payment now, part at the event.
4) Incorporate a referral discount. Incorporate previous clients/fans into your referral and reference program. Allow your customers to talk to new customers about the advantages of prepaying and your product.
5) Simply ask the client "what will make them feel comfortable with prepaying". or "what is stopping them from prepaying".
Sometimes we assume we know the reason for their hesitation, when it could be something very easy to accommodate.
I worked in the event planning space in the past, and now have one of the popular event planning blogs. You have to consider your target market first. Event organizers come in 2 different flavors - independent event organizer working for a client and an employee organizing the event for the company. Knowing which one is which during your sales pitch/conversation is critical because it helps you position your product aligned directly with event planner's goals and objectives to help them understand how your product benefits them, their client and event.
They're always on a budget, and a new product requiring an upfront investment without a clear benefit is a risk they are not willing to take to stay within their budget. You need to see what your competitors are doing (if you think you don't have any, than you haven't done your homework well enough) and position your product in a way that 1. it clearly solves event organizers pain/problem, and 2. it sets your product apart from the competition.
This really should be your value proposition, which is NOT the same as your vision/mission statement and/or your business description. It needs that ONE unique differentiator to set you apart from the competition and clearly communicate tangible solution to a pain/problem. Many businesses across industries miss this.
All in all, you are not communicating tangible, real benefits that solve event organizer's pain/problem to reduce perceived risk of an upfront payment.
Tip for the road: Don't use ambiguous language with event organizers, be as specific as possible.
P.S. If I had more details as to what your product does and why it requires an upfront payment (and how big it is), I could be more helpful in attracting and selling to event organizers - and understand what pain/problem it solves.