Perhaps a coworker writes to the office mailing list:

This Sunday I'm going to be participating in the Walk for Hunger. My goal is to walk the whole 20 miles, and am looking for people to sponsor me on a $/mile basis. All money I raise goes to a good cause: fighting hunger here in Massachusetts with Project Bread.

This is a socially acceptable way to ask your coworkers to donate money. Which makes it unusual: it's generally more acceptable to ask people to donate stuff via some sort of 'drive' [1] than it is to ask for money. Charities can do much more with money, however, so the question is, what makes this request work? Might there be a similar request that would convince people to give to more effective charities?

Contributing aspects:

• It's a request for people to sponsor *you*. You're not asking for them to give money directly to a large organization.
• You're promising to do something difficult. This indicates that you take this seriously and are willing to sacrifice for it. (That people afterwards describe events like this as "a wonderful experience" doesn't seem to apply.)
• The cause is mostly irrelevant, in that you could substitute pretty much any charity that sounds like it's doing something useful, but I suspect local charities and ones that are trying to fix some problem that affect people the donors know personally get more money.

[1] For example, some people at my work are currently collecting leftover halloween candy to send to troops overseas.