Things that make me willing to say “yes, I’ll serve on this committee”:

1) some indication of the timeframe. When will the busy period start, and when do you expect it to end?

2) some indication of the workload. What needs to be done during the busy period? How much paper—applications, proposals, whatever—needs to be read and processed by each committee member?

Admittedly, I’m junior faculty. Maybe you can expect more senior faculty to know these things already. But in my opinion, such details not only give me a sense of whether I can make this commitment, but they suggest the people I’m dealing with will be good organized people who make the process as easy as possible. And to those people, I’m happy to say yes.