The most obvious means of natural ventilation is a door or window that can be opened. To be most effective, ventilation needs a through draft, i.e. air enters via a window on one side of the building and is sucked out the other side.
Trickle ventilation is often used in newer buildings and is also often a feature of modern UPVC windows. Trickle ventilators are usually small holes in window frames that allow a small amount of air in at all times. This helps to reduce condensation, particularly during the winter months when the windows would not be opened.
Another form of natural ventilation is Passive Stack Ventilation (PSV). This works on a similar principle to a chimney, where air passing the top of the stack sucks air out of the building.