Make sure you're storing your polishes in a cool, climate-controlled area. It's also important to avoid storing them in a place that gets direct sunlight.
Store your bottles right side up. Storing them upside down can cause clumps and drying at the neck of the bottle.
Clean the neck of your polish bottles after each application. You can do this with a cotton ball and some polish remover. This helps ensure that the bottle will seal tightly when you close the lid to prevent air from getting inside.
Never leave the lid off your polish bottle for longer than necessary. Exposing the polish to air is one of the major causes of thick polish.