St. Joseph's Indian School helps Lakota (Sioux) children escape poverty through education. They have a list of things they need on their website and they update it regularly. So whenever you are getting rid of stuff, you can check the website to see if there is a need for it. Most of the time, it's everyday things you use at home.
Of course, they accept monetary donations and I personally have made a modest donation last year.
There are many things you might have lying around the house not being used like shampoo, pillows, scrapbooking supplies, kitchen utensils, etc. I have duplicates of scrapbook paper that I got in a book which I won't use so I think the kids could benefit from making projects with it. I just have to find a mailer that can hold 12x12 paper.
Here is the link to the list of supplies that St. Joseph's needs in case you are interested... http://http://www.stjo.org/site/PageServer?pagename=help_non_monetary_needs