NASA seems to have a mission planned to send a 2016 lander to Mars, and they say that they are in the process of choosing landing targets. What factors are considered when selecting these sites?
This presentation for Mars Exploration Rover landing site selection gives a good overview of the typical engineering considerations for a robotic Mars science mission landing site. This paper gives more details including the science considerations. MER is a good example for the consideration of safety, mission success, and science objectives for landing site selection.
You can find a large amount of material on the Curiosity landing site selection here.
InSight is a little different from previous landing site selections, though similar to the one for Mars Pathfinder in some ways. For InSight, the selection has fewer factors to consider, since the science mission doesn't care very much about the characteristics of the landing site. All they really need for the science is good contact with firm ground for the transmission of seismic signals. The InSight landing site selection is all about landing safety and solar power for the mission.
InSight recently narrowed down to four landing sites: Elysium, Elysium, Elysium, and Elysium. They're all in the same place since it is a uniquely large, flat, low rock abundance site, near the equator for good solar power. This paper provides many more details on that site selection.
The landing site selected based on two factors
The landing site must be selected such that it should be very close to the testing area (such as habitable zone) where experiments are to be conducted It include geological , mineralogical, and meteorological.
It is protect the rover because of the engineering constraints. Includes dust factors (Mars’ ubiquitous red dust can interfere with all sorts of machine parts and instruments, particularly optics which was a problem experienced in the last rover missions Spirit and the still working Opportunity), atmospheric conditions during landing, elevation of the Martian surface, surface roughness and inclination and finally solar surface.
Surface roughness and inclination is a major factor for missions especially those that use airbags to cushion their landings.