A better way to translate this verse from the original language would be “always be being filled with the Holy Spirit.” There are 2 questions I will address here: 1) How do you go about being filled with the Spirit? and 2) How do you know if you are filled with the Spirit?
How do you go about being filled with the Spirit? – Books have been written on this but I will give the short version. It starts with confession and repentance and a desire to allow God to be in control of your life. The issue here is “who/what is in control?” It also involves a daily surrendering of yourself to the Lord who has saved you (see Romans 12:1-2, Galatians 2:20). And then it takes disciplined intentionality to set your heart on things above and set your mind on the things of the Spirit – instead of setting them intentionally on the things of this world and the lures of the evil one.
How do you know if you are filled with the Spirit? – The 3 verses following verse 18 give some clues here. 1) There will be a song in your heart (v. 19). In other words you will notice a change on the inside. You will begin to experience a palpable joy. 2) You will find yourself becoming more grateful (v.20). You will recognize the goodness of God in everything and express this to Him. 3) Your relationships with people will be different. You will develop an others-first mentality rather than a me-first mentality.
Why this BRICK is foundational: Here is what Romans 8:13-14 has to say about it – “If you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live. For all who are led by the Spirit of God are sons of God.” It seems to be a matter of life and death. ‘Nuf said!