How can I extend the life of clear nail polish? I use nail polish in several of my crafting endeavors. Example would be its use in converting my ties to clip-on or even to stop ribbons from fraying. 

Recently I was making construction paper jewels for my kids and testing finishes. I pullout my clear nail polish that I use and it was tacky. It didn't make for a great finish and was hard to apply. I cleaned the brush with acetone to remove the gum from there but it didn't help.

Is there something I could add to the polish to thin it out or something I could do to give it more life? Yes, I could just buy some more but I am not a fan of waste.
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.