The biggest thing that we all can do is to protect the environment around us. Of course, you know of "reduce, reuse, recycle," but also stop consuming as much as well. The less we consume, the less that will be used. We do not need all the stuff we have, and the more we have, the more resources are needed to create, construct, sell, and store those items. Also, be mindful of what we do consume. This includes clothes, water, household items, etc.
We also can help them by donating money to or visiting wild animal refuges, which fund the preservation of these animals.
Try to minimize the use of things that kill wildlife, such as pesticides.
Also, stay educated on which animals are endangered, and seek information on how to help that particular animal. Some programs such as "adopt an animal" programs are great. There are other ways which are unique to individual species.