Landing large payloads on the Martian surface
Landing large payloads on the Martian surface is needed for the construction of a base or colony. This requires untested technologies.
Because Mars has a thin atmosphere it is possible to use it for aerobraking. However, the atmosphere is not thick enough to entirely stop a large re-entry vehicle of more than a few tonnes. As the vehicles velocity goes down, the aerodynamic forces are insufficient to sustain flight, and the vehicle crashes before it slows down enough for the use of parachutes.
The Mars Science Laboratory used an aeroshell, parachutes and a rocket powered "skycrane" to land on Mars. This may be used to land medium sized payloads on Mars with great precision. Larger payloads are harder to land, but increasing the size of the aeroshell may help. An inflatable Rogallo wing made out of inconel wire and silicon rubber was considered as a manned rescue vehicle from LEO, and so was a system called MOOSE, using a foldable heatshield as a one-man escape system. Other system to be considered include high speed ballutes.
If enough fuel is available in Low Mars Orbit then fully propulsive landing, as on the Moon, might be a possibility.
Main article Space elevator
A space elevator could be used to drop payloads onto the martian surface. A cargo pallet docks with the counterweight, and slides down the the cable to the surface. A space elevator must be built at or near the equator, whereas the cargo might be needed thousands of kilometers away. Airships could be used for long distance transport. Construction of a space elevator may require many "conventional" landings on the surface and a large amount of infrastructure in space if it is to be feasible. Even if a space elevator is implausible to construct a colony in the first place, it might be useful in future.
Supersonic Retro-Propulsive landing
SpaceX has developed a concept called supersonic retro propulsive landing that is designed to bridge the gap between aerobraking and propulsive landing. It appears this was tested in the Earth's upper atmosphere during the re-entry of some of the SpaceX Falcon 9 vehicles. Retro propulsion in the supersonic flight regime is used to support and slow the vehicle until conventional retro propulsion can be used to land. SpaceX has demonstrated the capability of very precise landing using these methods on Earth, that could be transposed to Mars.