A quick wiki copy and paste:
In passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design or climatic design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.[1]
The key to designing a passive solar building is to best take advantage of the local climate. Elements to be considered include window placement and glazing type, thermal insulation, thermal mass, and shading.[2] Passive solar design techniques can be applied most easily to new buildings, but existing buildings can be adapted or “retrofitted”.
In other words, design for your climate.