Someone in my Yeshiva has created a web page with links to many online stores that the yeshiva guys commonly shop at and when shopping through these links a portion of the purchase will be donated back to the Yeshiva. This way, people can help the yeshiva without spending any more than they were already planning to spend.
Many web sites (such as Amazon) offer this option of becoming an affiliate through their own web site and others (such as Diapers.com) do it through a service such as Google Affiliates. If you have some time you can try to set up a similar program for a tzedaks of your choice. Of course, it will only be worth your time if you can get a lot of people to shop through the links.
You can visit the site and help my yeshiva as well by purchasing through the links at http://tinyurl.com/yzdrsa