It seems that many people put their wireless router near their main phone line, or near a wall with a window, usually that is the location closer to the internet line coming from the street, and it is the most convenient, but it is not the best location for the wireless signal.
Placing the router on an edge of the house, closer to windows, make it much more susceptible to interference from neighbours wireless devices, while placing it in the middle of the house, make the cement walls act as a natural shield for wireless signals coming from farer locations.
It is also better to place the router about 2 meters of the ground, above most humans moving in the place.
It seems that the middle of the house, away from the windows is the best location, however routing cables for internet/power/ethernet seems to be a problem, if that is a problem for you too, try to find a middle ground.
If you have an experience with wireless router placement, please feel free to include it in comments.