I just read an earlier question about changing the orbit of Mars to be more earthlike. This was shot down because of the massive amounts of energy/time required.
So, if the problem has to do with the greater orbit, there is less energy being absorbed from the sun.
Perhaps we could create a large lens that would concentrate more of the Sun's energy on Mars. At a geostationary orbit around Mars, how big of a lens would be needed to capture the amount of energy necessary to simulate the amount of energy the Earth receives from the Sun?
If this is possible, perhaps the opposite could be used to reduce the energy going to Venus.