Luke 11:13 - "If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?"
The Holy Spirit is a gift (Acts 2:38). You cannot be good enough to earn the gift of the Holy Spirit, but you do have to ask. This is speaking of the baptism of the Holy Ghost, which is subsequent to the born-again experience.
The Holy Spirit resides in our spirits, and once He comes, He doesn't leave (John 14:16). There is an initial filling when the Holy Ghost first comes. However, His control and influence over our souls and body does fluctuate proportionally to how well we have our minds renewed to His will (Rom. 12:2).
In that sense, we can be more full of the Holy Ghost than at other times, although in our spirits the presence and power of the Holy Spirit does not come and go. Therefore, even after we receive the baptism of the Holy Ghost, there will be times when our souls and bodies stray from the leadership of the Holy Spirit and we need, once again, to be filled with the Holy Spirit.
In Ephesians 5:18 believers are commanded to "be not drunk with wine...but be filled with the Spirit." Being filled with the Holy Spirit is in the present tense, making it a continual command for the believer. In the book of Acts, the same people who were filled with the Holy Spirit on the day of Pentecost were filled again. Most people don't get drunk on just one drink.
Likewise, being filled with the Holy Spirit is not just a one-time experience. There is an initial filling of the Holy Spirit, but many subsequent fillings. Just as drunkenness can change a person's personality and make him or her act totally different, so being filled with the Holy Spirit can make us act just like Jesus. Be filled with the Spirit today.