Don't throw out your nail polish!!

I used to toss nail polish when it got thick and gloppy.  I used to pass on clearance nail polish that was separated and looked icky.  Polish that looked like it was "going bad".  Those were the dark days before I knew that nail polish never gets old.
 If your polish has separated, shake it. It will mix back together again.  Essie polish is notorious for separating.
If your polish is thick, add polish thinner.  Add it drop by drop, shaking to combine, until the polish is back to its normal consistency. I've used polish thinner to thin both nail polish and top coats. Top coats need something more on the order of squirts of thinner vs. drops of thinner.  Nail polish gets thick as the chemicals in it evaporate.  Thinner replaces the chemicals that have evaporated.    Never try to thin polishes with nail polish remover.  Polish remover is designed to break the chemical bonds in polish.  This is not something you'd want to add to your bottle of polish.
The nail polish thinner I have is Beauty Secrets Nail Polish Thinner from Sally Beauty Supply.  It is about 4$.  It is three free, if that is a concern.  In the past few years, most polishes have become three free.  I've read if you have older polishes, you may prefer Seche Vite Restore.  It contains toluene, one of the three chemicals that are missing in three free polishes.  The old polishes once contained toluene, so it makes sense that a thinner that contains it would be better for old polishes.  Seche Vite Top Coat contains toluene and Seche Vite Restore is supposed to work better for thinning is.  I thinned my Seche Vite Top Coat with my Beauty Secrets Thinner and it seems to be working just fine. I've read you can even bring completely dried up polishes back from the dead, although the quality may suffer some.
You do want to keep polishes away from heat and sunlight.  I've read this can cause the polish change color.