The first thing to be said is what Philippians 2:12 does not say? It is not advocating to work out your salvation to get saved or to stay saved. The context is extremely important starting at verse 1 through verse 12.
I'm going to paraphrase what's going on here. At verse 2-3, the Apostle Paul is telling the Philippian "BELIEVERS" to have the same mind, maintaining the same love and be united in purpose. Do not be selfish or conceited and regard other people more important than yourself.
Verse 4, Do not look out for your own personal interest but put the interest in others first. At verse 5,6, 7, Have the same attitude which was also in Christ Jesus, who, (always existing in the form of God), emptied or veiled the prerogatives/glory He had before the world was. John 17:5.
Verse 8, And being found in appearance as a man He humbled Himself by being obedient to the point of death, even death on a cross. At verse 12, the first two words are "So then," which means, since the above is true, work out your own salvation with fear and trembling. In other words, "WORK OUT THE SOLUTION OF YOUR PROBLEMS" of not being selfish, conceited and not putting others first etc." Why? Verse 13, "for (or because) it is God who is at work in your both to will and to work for His good pleasure."
And to expand on this verse 14, "Do all things without grumbling or disputing; verse 15, that you may prove yourselves to be blameless and innocent, children of God above reproach etc. I think the "fear and trembling" warning means, "Don't give God a hard time."
IN GOD THE SON,
bluto