What measures have been taken to preserve the african elephant population?

Why do some people want a ban on the sale of ivory while others oppose such a ban?

The African elephant population isn't that threatened any more, I don't think. There were to many and they were having to cull them to keep the population down in protected areas. They have definitely set up nature reserves and banned hunting, the ban on ivory trade is because it sells for so much poorer people or greedy people go out and shoot the elephants unsustainably, that is one of the reason for the population decline in the first place. People want the ban lifted so they can make money from the sale of ivory again.
Hope that helps

This was just on an episode of "Nature" (PBS). I agree with what u08dc4 stated and would add that there are serious efforts being taken to keep the habitats for the elephants (and other wildlilfe) by blocking further land development for the growing populations and land speculators. I would have to say though, preventing poaching is the biggest effort made. There are projects underway to ease the poverty, make poaching less attractive, but ivory is a tempting source of money for many.