The following is taken from a forum post created by Martinwil (aka RommelsBadAss). For more information, visit the forum by clicking on the link in the blogroll.
Bronies for Good is a newly-formed grass roots organization that embodies Brony values of friendship, love and harmony by organising various public service projects. As a relatively new organization, we’re happy to announce our first act of service for the broader community, a blood drive! To take part, just gather up some bronies and head down to your respective blood bank, hospital, or red cross equivalent organization and follow this handy guide from the Red Cross for donating blood:
http://www.redcrossblood.org/donating-blood/donation-process#t1
Keep in in mind that you may also be required to make an appointment for giving blood, but this varies from organization to organization.
In addition, if there is enough people willing to organize a localized full fledged blood drive under BfG’s name, check out this link:
http://www.redcrossblood.org/hosting-blood-drive
For LA Bronies:
http://gotblood.ucla.edu/body.cfm?id=50