There is an interesting relationship between asking people to volunteer time and give money. Asking to volunteer then asking to donate get more money!

It turns out that when people are primed to think about money they are more focused on achieving their own ends with the resources they have. They think more analytically and give more conservatively. But the way people think about time is fundamentally different from the way they think about money. Because people measure time in the experiences they have and those experiences provoke emotions, the Time-Ask puts people in a more emotional mindset and they give more generously