To keep your nail polish usuable as the first time you opened it. Keep it in the refrigerator. I keep mine in a plastic container w/lid. I use it to the very last without ever adding a thinner or discarding an almost full bottle.
I have worn Obsession since 1985. I wear it every day, day or night. I have always had people ask me what am I wearing. I have even had men ask then sayi they were going to get some for their wife. It speaks for itself.