Keeping nail polish in a cool place such as a refrigerator will help keep nail polish from thickening.
Nail Polish Remover
To thin nail polish that is already thickened, simply add a few drops of nail polish remover, put nail polish top back on the bottle and shake well. This will help thin nail polish out.
Purchase Nail Polish Thinner
You can purchase a Nail Polish Thinner made specifically for this purpose such as OPI Thinner if you find that thickened nail polish is a common problem.
Flip the Bottle and Warm it Up
Turn your nail polish bottle upside down to distribute color evenly throughout the bottle then gently roll the bottle between your palms to warm the bottle up. The warmer nail polish will spread more evenly.