You mean things like "women's clothes"?
Simples.

Add 's to a noun to get the possessive, unless it already ends in an S.

Boy - boy's
Boys - boys'
Girl - girl's
Girls - girls'
Child - child's
Children - children's
Man - man's
Men - men's
Woman - woman's
Women - women's
Person - person's
People - people's

There's no such word as womens, so womens' can't exist either.
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Tue 4 Aug 2009, 15:15, Reply)