Keep it personal - ensure that in your newsletters and on your site you have personal stories of people who benefited from your organization efforts, personal appeals for actions (be it advocacy, donations, volunteerism). This works better than simple statistics.
But whatever story you use, be sure it integrates well with your brand identity (like say civil rights, or homelessness) and that it has what every marketing program should include - benefits to the reader and a call to action.
1) Start a newsletter and invite folks who visit your site (or who already donated) to sign up.
2) Check out your website statistics and find opportunities where site improvements can lead to greater conversions. These are low cost ways to market to people who already interested in your organization.