Step One:

Request free pet food coupons (usually for new products, but not always) or find really good coupons that make things free or close to it in your paper. These don't have to be for brands you'd use, or even terribly ethical brands, as they're not making any money and you'll be donating the product, diverting resoruces from buying food to other needs and hopefully allowing more animals to be saved.

Step Two: Buy the food with your coupons, freebies, etc. Sometimes the companies will even mail small samples.

Step Three: Donate.

My local pet food store has a food and supply donation bin right by the front door, so I don't even have to take things I purchased to donate out to my car. The donated food goes to a local rescue and to a feral cat group. You can also take the food you get to your local shelter (check their policy first) or to your local food locker to give to families in need with pets. Your local battered women's shelter, if they take pets, may also need pet food. Another option is to keep some dog food (this works well for smaller bags, cans, pouches, etc) in your car and give it to homeless people you see with dogs- often they're feeding a decent share of their people food to the dog, so this helps both of them to get fed.

Free pet food deals I know about:

http://www.nutroproducts.com/yourfamily / This one will send you a coupon by mail. Say you have a dog and a cat, they'll send a coupon for each.

http://www.mightydog.com/home.aspx?sid=1&p... One free can. Might not be worth the trouble since it's only a buck.

http://www.bluebuffalocompany.com/sample/t...

Please feel free to add any other coupons-by-mail or printables you know about.