I think people miss the point, the term "work out" is actually a mining term, it means to take what you have been given, and craft it, Mine it, Make it worth something. Salvation is already yours. What paul is saying is craft your salvation in a way that is meaningful.
Salvation is yours. It is NOT saying work to earn salvation, it is saying take what you already have and make it worth something,, ie, work it out in a way it affects others. Bears fruit, and brings others to christ (as a light in darkness) abd be careful, because things we do can put out that light.