Include your allies, opponents and target. This list will also be handy down the road when you want to send out invitations to an event or a thank-you letter. Write a strong letter outlining who you are and why you are writing. Include similar information that you would include on your flyer. Or send the flyer too - maybe they will hang it up in their office, business or school.
Mass mailing costs money - copying, envelopes and postage.
If your group doesn't have funds to do this - consider hand delivering the letters. Or you may decide to send out your letter through email. Making presentations to various groups and organizations is another good way to get the word out. Contact groups that you think might be interested in your cause and ask them if they have presentation time available at their regular meetings. These can be social groups, church groups, service organizations, youth clubs, etc.