What is a good method for learning/memorizing Chemical Formulas?

make each letter stand for something for example CO2 is come on and make a story with all of them.it might take a while to think of words easy to remember for all of them but when you have your little story it should be easy from then on.it would be even easyer to get the words to rhyme.

Something like sulfate, sulfite and sulfide are systematic, and carry over into other polyatomic ions.

Sulfate = SO4,  sulfite = SO3, Sulfide = S

Chlorate = ClO3, chlorite, = ClO2, chloride = Cl

Nitrate - NO3, nitrite = NO2, nitride = N

Phosphate = PO4, phosphite = PO3, phosphide, = P

Now why didn't I give you something like -NO, -ClO ?  Well, nitroso and hypochlorite, but I didn't want to confuse you.  Some you simply have to learn.

Now, in the above examples, all anions, I have not listed the ionic charges.  Awkward to do in Y/A.