Question

what is an easy way to memorize polyatomic ions

Answer 1

Obtain an eidetic memory. Look at ions. Win.

Answer 2

Say for Example,
You have Phosphate Ion - PO4
then positive valency (tendency to lose electrons) is 5 of P.
negative valency of O is 2...
electron requirement for 4 Oxygen atoms would be 2*4 = 8 electrons
but P can only release 5 electrons..
So whole charge on PO4 ion becomes 5-8 = -3 (negative 3 charge)