Tighten up the top of your nail polish bottle and shake it well. Now pour hot water on nail polish bottle for few minutes to thin out your favorite nail polish. Or, you can also place nail polish bottle between your palms and rub it vigorously so that warmth will help to reduce thickness of your dried nail polish.
Store your all nail polish bottles in one drawer. Make sure that place has to be clean and dry. Storing of nail polish in dry place will decrease the thickening process.
Some people also use alcohol as nail polish thinner. Add few drops of alcohol and shake it well. It will not only make your nail polish thin, but it also makes nail polish shinier. Do not add ample amount of alcohol, just use it as you use paint thinner to thin your lumpy or dried nail polish.