One of the best programs that I have enrolled in is the Staples Rewards program. For every ink cartridge that you recycle, you receive $2 back in rewards (up to 10 cartridges a month). I especially love this program because I buy discount ink cartridges online and my printer takes 5 total. Last month was my best month ever--I got cartridges for under $2 each, recycled the old ones at Staples, and was allowed to use rewards to purchase 200 sheets of 4x6 photo paper. The paper was an easy rebate item, so when it was all done, I received a check for $18 to take home 200 sheets of photo paper. With a little planning and searching for sales and discounts, you can get a lot of school supplies for free or very cheap. The Staples Easy Rebate program is great and really earns its name. It can all be done online and the information included on the receipt. Overall, Staples is a great place to do business when homeschooling on a budget.
Edited to reflect changes in the rewards program--the $2 credit is only valid now if you've spent $30 on ink or toner in the past 180 days. I have not used this service since the change, but the program could possibly still be useful. I also recommend checking Staples in general for their specials because they are a great source for inexpensive and sometimes free paper or items with their easy rebates.