Biologists recognize that habitat destruction has the largest impact on endangering species (it is at the top of the famous “HIPPO” list). If we want to help maximize life in the Universe, giving it a decent place to live is a good start – especially if we took it away in the first place.
First we must preserve habitat, which mostly translates into stopping land “development” (such as burning trees to grow crops, building homes and industrial facilities, and diverting water from current paths). Pollution, especially the gases contributing to rapid climate change, should be eliminated because it makes ecosystems uninhabitable.
To restore habitat, land currently used for other purposes (previously removed from its natural state) can be adapted to be used by wild animals, preferably native ones. For example, the National Wildlife Federation has a number of suggestions for homes, and permaculture offers a thorough design approach to coexisting with Nature, with benefits for all concerned (see especially the book “Gaia's Garden”). Eventually, we will need to replace large parts of our “concrete jungles” with life-friendly structures that at least resemble natural habitats, and can be converted into actual ones.
We will also need to learn to live closer to each other, in that we will have to re-learn how to share – cooperating instead of competing – so we don't end up harming each other to keep our personal consumption at something like current levels. Placing a higher value on relationships will make this easier, as well as weaning ourselves from the focused-power consumer culture that demands that everyone try to have their own copy of everything.