Something else to keep in mind:

If a lawyer volunteers for an hour serving soup to the homeless his donation should be valued at minimum wage. If he helps set up a trust to stop the corrupt "directors" of the charity from helping themselves to the money both now and in the future, his hour is worth a whole lot more.

This is the same for any trained professional. Anyone who has to pause and ask him/herself, "does my insurance cover me if I donate my time doing X" has highly valuable time to donate.