Yes. Definitely pre-sell. Not only does this fund your development efforts, it is the ultimate validation that you have actually come up with a product that people want to buy. There's a big difference between people saying they would buy something--and actually pulling out a credit card.

Offer your "charter members" 10% or 20% off for life in exchange for a 3 or 6 month prepay. Reverse the risk by guaranteeing their money back if they don't love it.

If you can't get clients to pre-pay, then your offer is not compelling enough.