Not by effort, but by rest.
Watchman Nee states in "Sit, Walk, Stand" that we cannot advance in the Christian life until we rest in Christ. He says that all "effort" at the spiritual life is doomed to fail because of the flesh. However, he says that when we cease trying and instead trust in God to work in us and have faith that God already accomplished all things for us in Christ, then almost unconsciously we will begin to exhibit the divinity that lives in us via the Spirit.
The other approach I've seen to advancement in the spiritual life is through the spiritual disciplines: prayer, fasting, meditation, study, etc.
On one hand, I've seen the benefit of the disciplines. On the other hand, the disciplines are always undermined by the very thing they seek to master - the flesh (e.g., I fail to pray because my flesh is too lazy). This makes Nee's proposition particularly compelling to me. But it also seems dangerous to stop "trying" to improve at the spiritual life.