I just thought I would quickly address this topic, since I often hear people talk about it inaccurately. Too often, I hear people say that they had to throw away their polish because it was either "too old" or "too thick." Other times, I have heard people say that they added nail polish remover to their thickened polish. First of all, polish does not get too old and go bad. So there is no reason to chuck old polish. Some of the polish in my stash has been there for 20 years.
Second, polish that has thickened can be fixed very easily, but nail polish remover is not the way to go! Think about it...you use nail polish remover to remove your polish. It breaks it down. So adding it to thickened polish will not thin it. Remover will destroy it. Simply get some nail polish thinner (I use the Beauty Secrets brand from Sally Beauty), and add a few drops. If the polish is really thick, add a few more drops. Then close the bottle, and shake it like mad. Check the consistency, and add more thinner if needed.
Conversely, there is an easy trick to thickening up polish that is too thin and runny. Simply leave the cap off for 5-10 minutes, and check the consistency. Repeat as necessary. Enjoy the rest of your day, all!