The definition of what it means to be filled with the Holy Spirit is something that is, sadly, controversial among Christians today. Yet, the Bible commands the Christian to be filled with the Holy Spirit. However, from my experience with people that I have talked to, it seems that many Christians have an incorrect idea about what it means to be filled with the Holy Spirit. It seems that Satan has been successful in confusing many Christians in this area, because Satan knows that being filled with the Holy Spirit is where the power is at. Satan knows that a Christian, while they are filled with the Holy Spirit, is in complete obedience to God, has their thoughts completely on God, is not sinning ("So I say, live by the Spirit, and you will not gratify the desires of the sinful nature," Gal. 5:16), and is fulfilling God's will. Therefore, it makes sense that Satan should do everything he can to confuse Christians in this area.
I'm not going to speak from some doctrinal standpoint here, or from something I read. I'm going to speak from my own personal experience. I have been filled with the Holy Spirit a number of times in my life (though, ideally, I should have been filled with His Spirit on a consistent, regular basis). From experience, I know that becoming filled with the Holy Spirit (the Third Person of the Trinity) does not come by praying for it or by asking for it. It comes by spending intimate, personal time with God---spending time in God's Word, in prayer, and/or praising Him. There is no exact 'formula' for becoming filled with the Holy Spirit, other than spending time with Him. I have spent periods of time reading and/or studying God's Word, and was not filled with His Holy Spirit. At other times, I spent time reading the Bible, or studying the Bible, or praying, or singing the Psalms, or praising Him, or a combination of those---and I was filled with the Holy Spirit. Every time that I was filled with the Holy Spirit, it was accompanied by joy and by a feeling of peace, along with a feeling that I was acting in accordance with God's will. If I remember correctly, the times that I was filled with the Holy Spirit were times of trouble or distress---in other words, in time of need. That's probably because those were the times I sought after God the hardest and the most sincerely and deeply. I also know, from personal experience, that the very second that you sin, you quench the Holy Spirit, and you are no longer filled with the Holy Spirit. Every time that happened, I could feel it immediately, and very obviously. I could feel that I was no longer filled with the Holy Spirit.
Sometimes when I was filled with the Holy Spirit, I became so ecstatic that I would lift up my arms in praise, and just verbally praise His Name. Other times, I would yell out in excitement. These were merely outward ways of expressing how I felt. At such times, the joy that filled me was beyond any earthly happiness or contentment or excitement or fun that I have ever felt. A couple of times, I have been filled with the Holy Spirit in the midst of hurtful tragedy. At such times, although I was almost overwhelmed by problems and conflict and others coming against me, I felt a deep contentment and had strong inner confirmation that I was doing the right thing in God's eyes. When a Christian is filled with the Holy Spirit, they are in perfect communion with God, and perfect fellowship with Him. When a person is filled with the Holy Spirit, nothing matters to them except God. When a person is filled with the Holy Spirit, they have the mind and attitude of Christ ("Your attitude should be the same as that of Christ Jesus," Phil. 2:5).
The filling of the Holy Spirit is not something that just happens once. It has to be continually sought after. Every time that person sins, they are no longer filled with the Holy Spirit, and they have to seek after it again, by spending time with God again. It is a continuous appropriation from a continuous supply from God. The Bible tells us to be constantly controlled by the Spirit. And the only way to obey God perfectly is to be filled with the Holy Spirit. When a person is filled with the Holy Spirit, they will only speak the truth, and their words will have power. The filling by the Holy Spirit is God's enabling that person to obey Him and serve Him.
A very imperfect and flawed comparison or analogy can be made by thinking about eating. When you are completely full and satisfied, you might think of that as being something like being filled with the Holy Spirit. However, as soon as you sin, you quench the Holy Spirit and are no longer filled by Him. Therefore, you might compare (please pardon the graphic description) throwing up to sinning, in that respect. As soon as you throw up, you are no longer full, because your stomach has been emptied.
The Holy Spirit never leaves the born-again believer (because God has promised that He never will), but when that believer sins, the Holy Spirit, though He still abides (lives) within that believer, He doesn't fill that believer. A cup, for example, may have water in it, but that doesn't necessarily mean that the cup is completely full of water. As another example, you may have some jewelry that is 10k gold, 14kgold, 18k gold, or 22k gold. But none of those are pure gold. Karat grade is used to express the proportion of gold in an alloy or the quality of a gold alloy. Fine gold (pure) is 24 karat. Pure gold will not rust, tarnish or corrode. Being filled with the Holy Spirit might be compared to something being pure gold, as opposed to the Holy Spirit merely indwelling the believer, which might be compared to 10, 14, 18 or 22 karat gold. Of course, unlike being filled with the Holy Spirit, karat grade does not change in jewelry, so the analogy is an imperfect one.
Every born-again Christian has God the Holy Spirit dwelling inside that person, and, because of that, the person's body is God's temple. The Holy Spirit can only abide in that person because they have been made holy by the blood of Christ. God will not dwell in a corrupted body that has not been redeemed. Being filled with the Holy Spirit simply means that you are completely filled with Him, and you are being completely controlled by Him. The evil counterpart is a person who is possessed by a demon. With the Christian, the person spends so much time with the Lord, feeding on God's thoughts, and surrendering himself/herself to God's will, that the indwelling Holy Spirit fills every part of that person, and every word, every thought, and every deed done by that person is controlled by the Holy Spirit. That person is completely under the control and influence of the Holy Spirit. They are of the same mind with God, and are humble but yet bold. They pray with the heart and vision of a righteous man. Their thoughts, ambition, behavior, and words are occupied completely with Jesus Christ.
Being filled with the Holy Spirit does not happen automatically. We have to spend time with Him. And being filled with the Holy Spirit is the greatest need, and the greatest resource, for the born-again Christian. When a person is filled with the Holy Spirit, they will always glorify Jesus Christ, and God will be exalted in their speech and in their behavior. If a Christian is sinning, they are not, at that time, filled with the Holy Spirit. It is absolutely impossible to sin and be filled with the Holy Spirit at the same time; just as impossible as it is for God to sin.
Another misunderstanding among Christians is what the 'anointing' is. Actually, it's really simple. When a person accepts Christ and becomes born again, they are anointed by the Holy Spirit, and the Holy Spirit then lives in them, and the Bible promises that He will never leave them. Simply said, a person is anointed if the Holy Spirit lives inside them.
"Christ" means 'the Anointed One.' An unsaved person is without the Anointed One, and is therefore without the anointing. But a born-again believer is in the anointing of the 'Anointed One.'
The Bible compares the Holy Spirit to an engagement ring, which is God's promise that believers will be united with Him in Heaven for eternity. The Bible compares the Church to a bride, and Jesus to a groom.
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law. (Galations 5:22-23)
(IMPORTANT NOTE: Although this is changing the subject, please note that the Son of God is not the Holy Spirit, and the Holy Spirit is not the Son of God. The Second Person of the Trinity is not the Third Person of the Trinity. The Father did not die on the cross. Neither did the Holy Spirit die on the cross. However, neither are they three Gods, as Muslims often falsely accuse Christians of believing. They are not a council of Gods. The Father, Son and Holy Spirit are three distinct Persons, making up one Being, Who is God. With people, three persons equals three beings. But with God, three Persons equals one Being. God is not like man in this respect.)