These answers are very interesting here.
Paul taught that Holiness is practical... but charged every Christian to understand.. that their responsibility to others was far more important than one's own personal feeling that anything I do or anyway I look is ok no matter what. "God looks on the heart".
While that is true... my humble opinion is this:
Paul said "All things are lawful for me, but all things are not expedient. All things are lawful for me, but I will not be brought under the power of any."
Paul is teaching that while it is legal for me to do anything within the newly given freedom in Christ...it is not always to my advantage. This is what is meant when Paul said "but all things are not expedient". So I must be careful what I choose to do...it could slow my advancement in the Lord. Expedient also means...not only that it might not be to my advantage...the greater weight would fall on the importance of how my freedom was affecting others. It might not be an advantage to others of my influence that I express myself in a particular way. The life of a christian is to be increasingly learned... that it is not about me... but it is about others...not about me...but it is about the team. And the greatest team of all is "my God and I".
So Paul would say "I have become all things to all men that I might save some." Always about others. This would be the reigning thought of God as it relates to man. God would be constantly becoming until He became our sacrifice for sin...and not stopoping there...but still becoming after the ressurection for our eternity. Always about us in His mind. Christians become. Paul does not mean, though, that he would become a prostitute to save a prostitute. Paul would strive to become her need. It is to the advantage of another to become as weak so that they can be made strong.
When God says "He looks on the heart" means that God sees the intent of the action... the reason that you are doing it. Man can't see your intent. Man can only look upon your outward appearance to make a valued assesment. He has no choice. He looks at the attitude that you express; your behavior; your actions and yes...even your dress. Is your dress appropriate for the place, circumstance or situation. Is it practical for your job. Holiness is practical but never offending on purpose just to make a statement. You could dress from the top of your head to the soles of your feet like some of the Pharisees did in Jesus' day...but if your personality stinks, and your actions are to injure, you are not expressing Holiness. Jesus never onced condemned their dress. He condemned their heart...their intent was to elevate themselves by putting down others in the name of God. The people couldn't see their intent but God did. The people only had at their disposal the outward appearance and the Pharisees blew a fabulous opportunity to lift burdens from off of the people.
God cares about your heart as well as the outward appearance... because both are representing Him. The only view of God that people have...is you. The only God they will ever know.....is the God... in you.
Hope this helps in addition to what has already been posted.
DMP