perchlorate - ClO41-
chlorate - ClO31-
chlorite - ClO21-
hypochlorite - ClO11-
If you look at te chemical formula and charge of the polyatomics, the only difference between them are the number of oxygen (the charge remains the same).
per - stem of word - ate - 1 more than -ate (highest number of oxygen)
stem of word - ate - one higher than -ite (this is one I memorize) and link to rest.
stem of word - ite - one less than -ate
hypo - stem of word - ite - one less than -ite (lowest number of oxygen)