Some SciFi crank writer proposed an interesting idea. Once a government decides that population control is necessary, it starts issuing newborn females with 'birth rights', number to be determined by the government at any given time. Might be something like 3, 2.5, or 1.8. This female is then entitled to bear that number of children. If she goes over her quota, she's sterilized (temporarily, of course). What makes it more interesting is that the birth rights can be bought and sold on the open market. A family who's female had 2.5 birth rights and has had two children wants a third child, they can buy .5 birthrights from a similar female who has had two children and decided that two is enough.
Interesting ramifications if you think it through...