Abundance and variety are key. Water can be provided with a simple birdbath or an elaborate, manmade pond. Even the overturned lid of a trash can will suffice. Just remember to break up the ice on cold days. Shelter can come in the form of evergreens, briars or thickets and trees with overhanging branches.
Birdhouses work too.
Use non-reflective window coatings and screens whenever possible. Or, flash tape, bird netting and window awnings help to deter the birdseye illusion. Migratory songbird populations can be saved, but it's going to take the work of several entities, from the U.S. Fish and Wildlife Service and other agencies down to the individual level. As citizens of the earth, we owe it to future generations to safeguard our endangered species, just as we do our natural resources. All in all, it's going to take a village to save the songbirds.