There are, at least, two different perspectives that can be derived from the phrase "work out your own salvation"...
- Do something to gain a salvation that you do not already have
- Live out the salvation that you already do have
Reading Phil 2:12 in context of its preceding verses has me to believe that perspective 2 is closer to what Paul is saying compared to perspective 1.
The preceding verses, Phil 2:9-11, we read that Jesus
- was highly exalted by God
- given a name above all others by God
- every knee will bow to Him
- every tongue will agree that He is Lord
In verses Phil 2:9-11, it appears that Paul is attempting to get his readers to reflect on how awesome, powerful, etc that Jesus Christ is.. the terms that Paul uses in 2:9-11 to describe Jesus should cause the believing reader to reflect that Jesus wasn't just a good guy, but rather the one and only being that all else is subject to his power.
With that in mind, Paul continues on by saying - because of who Jesus Christ is, you need to "work out your own salvation with fear and trembling".
In light of Phil 2:9-11, it appears safe to infer that Paul is not trying to stress that we need to "earn a salvation that we don't already have", but rather we need to work out what we already do have (in Christ) with fear and trembling - as if to imply that due to the fact that Jesus Christ is the one that every knee will bow to, live out the salvation that you already have with a mindset of awe toward Christ.