Although God wants every believer to be filled with the Spirit, many Christians are not sure what this means or what it looks like. To help us understand that whatever fills us controls us, Paul cites drunkenness as a negative example of "filling" and tells us to avoid it [Ephesians 5:18-21]. Every believer is indwelt by God's Spirit, but the extent of His rule is determined by the Christian's freedom to comply.
Think of this as a voluntary choice to surrender your life to the Holy Spirit's control—in other words, to be sensitive to His leadership and guidance, obedient to His promptings, and dependent upon His strength. The evidence of the Spirit's control is revealed in a person's character. Those who have yielded their lives to Christ's leadership are continually being transformed into His likeness. The degree of surrender determines the level of transformation.
Even though good works and faithful service are a result of being filled with the Spirit, they are not necessarily signs of it. Remember, we are talking about character, not what we do. It's easier to serve the Lord in some manner than to love the unlovable or be patient with difficult people. But when the Spirit is in charge of our lives, He does through us what we cannot do for ourselves.
Each believer decides who rules his life, by either actively surrendering to Christ or deliberately going his own way. Even those who try to avoid the issue by making no choice at all unknowingly opt for self-rule. The fullness of the Spirit and godly character await those who choose God over self.
Taken from "What Is the Spirit-Filled Life?" by In Touch Ministries (used by permission).