They could offer a subscription model, and then make it a monthly "game" of deciding where your money's going to go. Lots of people are willing to become sustaining members of charities.

Or, I'm a member of a meta-charity called Foundation Beyond Belief, in which my contribution is distributed to various nonreligious charities based on priorities I set (e.g. 25% of funds to environmental causes, 50% human rights, 25% poverty, or whatever). So for Watsi, maybe I could predefine, say, types of medical treatment, country/region, etc. and then the system heuristically determines where to disburse.

And now that I think about it, Kiva optionally does something similar, so that loan repayments don't just sit stagnant in people's accounts because they haven't gotten around to manually distributing them.