Philippians 2:12 "So then, my beloved, just as you have always obeyed, not as in my presence only, but now much more in my absence, work out your salvation with fear and trembling"

Paul is not contradicting himself here and teaching a "works" salvation, but this passage must be read in light of its context.  Paul had just finished talking about the obedience of Christ, who was obedient even till death on the cross.  He then exhorts them in the verse quoted here to obey, and to continue to obey.

A mark or fruit of those who are saved is demonstrated by their obedience to Christ with fear and trembling.  We are not saved by our good works, but by faith (Ephesians 2:8-9), however in Ephesians 2:10 we read "For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand so that we would walk in them."

The sign of a true believer is an obedience to Christ, and an obedience that endures to the end. (Matthew 24:13)

Note: The NASB also cites Heb 5:9 and 2 Cor 7:15 as cross references for this verse that also help with our understanding of this passage.