How can I extend the life of clear nail polish?
Some people use acetone to thin down their nail polish to its original consistency, but this is not recommended since it will cause it to dry out more quickly after application and can cause cracking in the finished result.

There are a number of products available on the market; they are intended for extending the life of normal nail polish but I'm not aware of any reason they wouldn't work for clear nail polish also.

You need to search for 'nail polish thinner'

Also, be aware that shaking the bottle or pumping the brush in and out will cause it to thicken more quickly.  Ideally you should roll it between your palms before use instead.