Conservation of what one uses is the first place to start. Where I live, I have a well that supplies water and a septic system that puts the waste back into the ground where it is purified by natural actions. Our use of water per person is low when you look at what is kept out of the "Stream" of chain of events, maybe less than a few pints a day.

Most cities have the water piped in from miles away, then pump the sewage miles the other way which has no long term resorce for replenishing the supply verses use. Or the sewage is sent to a large sewage treatment plant that "Cleans" it and dumps it into a river which carries it even farther from the source. In some areas of America, the total water table has dropped to a point that accessing water is getting to be an issue and areas farther out are being tapped to repeat the cycle.

Food supply has turned into a Global market, but there are safety issues to be concerned with. Some places put untreated human waste directly out in the fields for a water souce and that is a source for E-coli and other nasties. With various amounts of that food grown sourced into the whole world market there is a certain risk involved. Buying locally produced food at least gives you an idea of what conditions might be. This does nothing though for the third world countries that are trying to be in the Global market to support their farmers and the like.

Having your own garden is a great way to go, but most folks don't have the time or space to do so.