One of the most accurate facts one can ever reference is statistics. So long as these numbers are gathered in a way that is scientific and truthful, they can accurately represent a situation such that it appeals to the generosity or compassion of a person. For example, you could cite the number of abandoned animals that must be killed off in shelters every year due to a lack of funding. Using statistics to sell your cause at your charity garage sale can be a very effective tactic.
You'll need to start out by gathering accurate statistics on the problem or problems. Head to the web, but don't trust every number you run into; there are people whose intent is to maliciously corrupt the free information available online. You can also contact the organization -- or a similar one -- that you're planning to help, to see if they have any data that could be useful for you. Keep in mind that every source you come across will have some sort of bias in what data they provide, so try to compile your results from several sources.
Pare down the information you get so that only the necessary facts are there. People can easily become overwhelmed when a ton of numbers are thrown at them, and because numbers can represent something in so many different ways, it's easy to get carried away when you're compiling some data into an informational packet. Go over each statistic you plan to list and decide whether or not it is absolutely necessary to your cause.
Represent the data in a way supporters can understand. Many people can't wrap their heads around a bunch of ordinary numbers, so if you can compare it quantitatively to something familiar and easy to understand, the numbers will become more realistic to them. For example, instead of saying how many pounds of food are wasted per year, cite it in the number of complete fast food meals that would be equivalent.
Most importantly, don't over-use numbers. Again, it's very easy to get carried away, but if all you're using is numbers to support your claims and causes, people will start to doubt that there is any real humanity to the cause. Bring in the faces of people or animals (or places, even) that have been affected by what you're trying to change, so that there are real stories behind the lofty concepts you'll be asserting.
Need a place to list your charity garage sale at no cost? Don't worry, we've got you covered over at our home page; come check it out when you would be putting listings in your local paper.