Philippians 2:12
12 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
The above verse is among the most common verses in the Bible used to teach works Salvation, because it uses the words "work" and "salvation" in positive conjunction.
By those who would use this verse for such a purpose, this is seen as an instruction to "work for your Salvation", all the while "trembling with fear" that you may in fact go to Hell when you die.
Is this a valid interpretation?
First, let's examine some other times the Apostle Paul uses this phrase "fear and trembling" to get a better sense of what he's saying.
We see him use the phrase in First Corinthians chapter two to describe how he presented himself amongst the Corinthian church:
1 Corinthians 2:3-5
3 And I was with you in weakness, and in fear, and in much trembling.
4 And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power:
5 That your faith should not stand in the wisdom of men, but in the power of God.
Here, we see that "fear and trembling" refers to the humility that he presented himself with to them, so that their faith wouldn't be in him as a powerful orator, but instead in the Gospel (Romans 1:16) that he was preaching.
He uses the phrase elsewhere to the same effect:
Ephesians 6:5
5 Servants, be obedient to them that are your masters according to the flesh, with fear and trembling, in singleness of your heart, as unto Christ;
2 Corinthians 7:15
15 And his inward affection is more abundant toward you, whilst he remembereth the obedience of you all, how with fear and trembling ye received him.
We see that these are references to humility and meekness, not terror. Therefore, our verse in question is exhorting the Philippian believers to conduct themselves in humility, not tremble in terror.
But, does the accusation still stand? Is the verse in question a directive to work for our own Salvation with humility?
We can know off-hand that whatever is at stake here, it is not the eternal Salvation that is obtained by faith in Christ, as we are told elsewhere that that Salvation is "not of works":
Ephesians 2:8-9
8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
9 Not of works, lest any man should boast.
Titus 3:5
5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;
Above, we have two clear witnesses to the fact that we are "saved" from our sins (Matthew 1:21) without works, which are but two of dozens and dozens of verses teaching such a thing.
Therefore, we can see that the "salvation" being spoken of in the verse in question refers to temporal deliverance, in much the same way it is used in the previous chapter:
Philippians 1:19
19 For I know that this shall turn to my salvation through your prayer, and the supply of the Spirit of Jesus Christ,
Of course, above, the Apostle Paul is not talking about getting to Heaven because the Philippians are praying for him. This temporal sense of the word "salvation", meaning an earthly deliverance, is used many times in Scripture (Matthew 14:30, Romans 13:11: 1 Timothy 2:15, 4:16: 2 Corinthians 1:6, Acts 27:31, etc.).
In closing, this verse does not teach that, contrary to hundreds of verses of Scripture, we must work to earn our Salvation, "trembling" with horror the entire time. That is a serious twisting of the message this verse is trying to convey.
Rather, it's a simple exhortation for believers to conduct themselves humbly, and thereby secure the blessing of deliverance from the Lord.