Don't throw away old nail polish! No matter how goopy and unworkable it is there is a way to fix it. Simply get nail polish thinner and your old nail polishes will be good as new.This will save you money since you don't have to go and buy all new nail polishes.
You use nail polish thinner by adding drop by drop until nail polish is the consistency that you would like it. Make sure you mix it up, a good way to do that would be to roll the bottle back and forth between your hands.
There are so many brands of nail polish thinner that it can be hard to choose. I would recommend using OPI Lacquer Thinner or Seche Restore (Top Coat and Laqcuer Restoration Thinner)
(Images from amazon.com)
Both of these have great ratings. OPI is the leader in quality nail polish with a trusted quality with all of their products. Seche Restore has just as great reviews and it comes with a dropper so you can control the amount you put in a bottle of nail polish.
I have ordered a bottle of the Seche Restore, I have a few nail polishes that really need thinning. I will be doing a review of this when I finally recieve it, after I use it.
A little note I think I should add: It doesn't matter what the brand of nail polish thinner, you can use it on any nail polish. You don't need to run out and get a certain thinner for each brand. Any brand that you chose should work with any brand.