Can You Handle the Uncertainty?

Accountants make a "maybe" complicated

When a person asks if I would like to attend an event, or donate money or time, or make any other kind of commitment, maybe does no one any favors in lieu of a yes or no. My interlocutor will make a note of the answer and come back to me at a future point; all I'm usually doing is putting off a "no" answer so as not to hurt the person's feelings. It does sometimes happen that more facts need to come in (e.g.,"my boss might need me to go out of town") before I can commit, but that's rarely the case.

However, a "maybe" can increase the likelihood of a "yes", and that's when it's offered by the requestor. [bold added]

You might consider asking people for a “maybe favor.” A maybe favor is a request for a commitment that might not actually have to be carried out. In your case, you could call for volunteers while making clear the possibility that the clean-up will be cancelled in the event of rain.

Recent research suggests that adding a “maybe” to a request for a favor increases people’s willingness to help. When subjects were asked if they would be willing to donate their earnings from participating in the study, 53% agreed. A different group was asked the same question but told that 5% of those who agreed would have their donations randomly cancelled. Under this condition, 66% chose to donate, which increased the total value of the donations even after eliminating the 5%.

One possible explanation for the increased willingness to donate is that we value the “warm glow” we get from agreeing to help. If we think that there is some probability that we will not be asked to do the favor, the warm glow remains, while the cost of doing the good deed is potentially mitigated.

Asking for volunteers is difficult, but through personal experience I've found that there's less resistance if a person is asked to be a backup. ("I've got the five we need but just in case one doesn't show, can you substitute?")

Make sure it's really a "maybe", however. If the contingency never occurs and the possible always becomes a real obligation, then you've lost personal trustworthiness, which is more valuable to you than any ephemeral cause you may be supporting.