God's working in you for His good pleasure

Therefore, my beloved, as you 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; for it is God who works in you both to will and to do for His good pleasure. Philippians 2:12,13 NKJV

The key to applying this passage is knowing who is doing what. Thankfully it's God's job to work His will in us and our job to put our faith and trust in Him. I work out my salvation each day simply by spending time with Him and building my faith and trust in Him. John 6:27 says not to labour for food which perishes but to labour for that which endures to everlasting life and is given by Jesus. When asked what work we should do, He said to "believe in Him".

I guess believing/trusting is called working out our salvation because it doesn't come naturally to us, and therefore requires effort, not because we are earning something. That's probably why we shy away from using the word work when talking about salvation. We know it's God's grace that has saved us, not anything we do/could ever do ourselves.