Most nail polishes should have a shelf life of 1 year.
So to prevent polishes from drying up and getting thickened polish...
1) Keep it in a dry and cool place. The best place to keep your polishes are in the refridgerator.
2) When storing your polishes, put it in a upright manner to prevent the solvents from separating.
3) Ensure that your bottle is tightly closed. This is a important step to prevent the polish from drying up.
You can also
1) Put thinner(a few drops) to your nail polish.
2) NEVER PUT NAIL POLISH REMOVER to your nail polish as nail polish remover contains water and is not compatible with nail polish.
I hope this tip helps:)