The universe is infinite, or seems very near it by our perception, so what matter are our individual actions? To avoid a depressingly nihilistic viewpoint (as opposed to the celebratory viewpoint that we can't screw it up all that badly), we accept some perspective and suppose that our goals should focus on the human scale. In the immediate future, we take this to mean the Earth and its immediate surroundings, potentially out to a distance of Mars, but not for the purposes of this discussion.
Even at such a reduced scale, we observe that there are just shy of seven billion people on this planet and that the actions of national governments, large corporations, and international coalitions dominate the global stage. This mode of thought leads many to question the importance of their personal contribution to environmental, political, and other movements. Such a point of view, however, is inherently flawed, because it is precisely this sort of incremental action that creates the current atmosphere.
We might point out that it is precisely because of individual choices accumulating that there is so much waste in landfills and that so much new technology is bought and scrapped so regularly, among other examples. But many are not convinced of this argument.
More effectively, we can illustrate the cognitive dissonance inherent in supporting some concept theoretically but not putting it into practice oneself. In so doing, we convince those who agree that water resources should not be wasted or that companies should try to curb emissions to reduce their own superfluous use of water and individual vehicular transportation.
Convincing a friend to act on such beliefs, or to vote his or her mind in an election, signifies cultural change. So long as we all ignore our knowledge of what is "right" (e.g. reduction of waste), we can say, "Well, nobody else is doing anything about it personally, why should I sacrifice my comfort?" If, as we realise that little discomfort arises from some of the changes we might implement, we enact our beliefs, our example says to others that not only motivated activists work for things like environmental issues.
As those willing to start this minimal action begin, their peers and communities will take notice and realise that people do care and that all our minute individual contributions add up.
In short, live your credo and lead by example, for actions speak louder than words.