Paul writes to the church at Philippi: Therefore, my dear friends, as you have always obeyed--not only in my presence, but now much more in my absence--continue to work out your salvation with fear and trembling, for it is God who works in you to will and to act in order to fulfill his good purpose. (2:12-13, TNIV)
Paul here brings forth an important element of spiritual (re)formation that we must understand as we strive to become more like Christ. The process of spiritual (re)formation is both active AND passive. Notice what Paul says: [you] continue to work out your salvation with fear and trembling. Here Paul is reminding his beloved family in Philippi that a tangible element of our salvation is that we are "working it out", that we are striving to grow. But, in the same sentence he offers the counterbalance: for it is God who works in you to will and to act in order to fulfill his good purpose.
The important lesson to be learned is this: Being conformed to the image of Jesus is an intimate, cooperative process in which we work to put ourselves in a position for God to work. It is not about being super spiritual or doing all the right spiritual disciplines (though these are good and helpful), but ultimately it is about making ourselves vulnerable and available to the God of the Universe working in us and through us.