An easy way that anyone can help is to donate blood through the American Red Cross. You can find your local Red Cross by calling 1-800-GIVE-LIFE or using the online directory. The Red Cross advises donors to call ahead and make an appointment instead of going directly to the local hospital or center. (Note: The Red Cross site may be inaccessible at times due to high traffic.)
If you are in New York City, call the New York Blood Center at 1-800-933-2566. The New York Times offers a list of blood donation centers set up around the area.