Some activities that each individual may have in mind in order to prevent the occurrence of large scale negative environmental impacts are, for example, saving water when washing the dishes or taking a shower, preventing the excessive consumption of energy when it's not strictly necessary, to separate the organic wastes from inorganic ones, decrease the use of automobiles to reduce the use of gas and petroleum, to use products of organic origin and with biodegradable packaging, within others.
All these actions, on a large scale, i.e a big proportion of the human population, would make a truly positive impact on the environment and therefore, prevent the negative ones.