Raise male wages. Most prostitutes must forego or postpone marriage, because most men do not want to marry a prostitute. The more appealing the marriage prospects, the lower the relative return to prostitution. Andrew David Chamberlain gives more analysis, and links to the relevant research.
Thanks to Michael of www.2blowhards.com for the pointer. I”ll also note I enjoyed his recent survey of conservative and libertarian thought in the blogosphere, not the least because he had nice words for Alex and me.