Go through your cosmetics kit and find a bottle of nail polish that you do not use anymore because of its thickness. Tightly hold the bottle in your hand and give it a vigorous shake. Your aim is to dislodge the ball that is put in almost all nail polish bottles for the sole purpose of keeping the nail polish thoroughly mixed. If you are successful at dislodging the ball, your chances of thinning out the nail polish will increase dramatically.
Fill an eye dropper or any other liquid dropping mechanism with acetone nail polish remover. Add a drop of nail polish remover from the eye dropper to the nail polish you are trying to thin out and shake the bottle vigorously. Add one more drop and shake the bottle again. Continue in this manner until you have added about 7 to 8 drops of nail polish remover to the bottle of nail polish on which you are working.
Check the nail polish to determine if it is of the desired consistency. If not, add more drops of acetone nail polish remover until the nail polish glides on nicely. Although this quick fix usually works in most cases, you will probably have to eventually discard the nail polish because it will go back to its old thickness when the nail polish remover vaporises with the passage of time.