Even if Earth threw all of its resources at sending people on one way trips to Mars, we wouldn't even be able to keep up with our current population growth rates. Currently we're experiencing ~1% growth, so in order to decrease the population of the Earth only by sending people to Mars we'd have to be sending almost 200,000 people a day, which seems extremely impractical.
It would have to be several orders of magnitude cheaper to get people to Mars than it is with our current technologies to have a chance at reducing earth's population in this manner. In fact, I haven't seen any sort of proposal for a cheap way into orbit that is believably scalable to that magnitude of people. Maybe something like a few space elevators would allow 1,000 people a day to head off for Mars, which would allow for a large city (or several small ones) to be built very quickly, but humanity isn't even close to being able to pull off an exodus like you suggest.
I think that practically, the best way to decrease the population on Earth is to do what China's doing with population controls, though it would really be better if people would just self regulate, rather than having the government get involved with that.