Taking care of vintage (and contemporary) costume jewelry so that it will last is very easy.
1. Always apply makeup and perfume before putting any jewelry on. The chemicals in the makeup and perfume are the most common reason for rhinestone and metal discoloration.
2. Store your jewelry in a dry place. Wetness can cause verdigris (that green gunk you commonly see on old jewelry) to form. Verdigris will literally wear away at the plating over time and can spread to other pieces. If you see a piece that has verdigris separate it from jewelry or put it in a ziplock bag so the verdigris can't spread. Wetness can also wear away at the foiling on the back of rhinestones causing discoloration and speckling. In other words, taking your jewelry off and leaving it next to the sink or jumping in a pool with your jewelry on aren't good ideas.
***Tip: You can use a little bit of vinegar to clean verdigris off of metal, but make sure not to get the vinegar on any stones as it can damage them. However, I do not recommend knowingly buying jewelry with verdigris, because most of the time those pieces have damage and the verdigris is a headache to remove.
3. Storing Jewelry: Try to store necklaces flat or spread out so that they don't break or rub. Make sure that all your jewelry is not thrown together and rubbing. My mom once threw an enameled Matisse necklace into her jewelry box in a jumble and the links rubbed and damaged the enamel. (Contrary to what some dealers will tell you, enamel is rarely worth the money to repair and never looks the same as the original.)
To conclude, in order to care for your jewelry, put on makeup & perfume before donning your costume jewelry. Make sure that your jewelry doesn't get wet or is stored in a damp place. If any pieces have verdigris keep them separate from other jewelry. Also, make sure that it is not stored in a way that pieces might get damaged through rubbing or being twisted into unnatural shapes which might cause it to break.