"Thou dost desire truth in the innermost being, and in the hidden part Thou wilt make me know wisdom."
The world is tired of Christians with externals only . . . strict adherence to dress code, numerous restrictions in activities, diet, etc. Convictions are fine and provide some security. But they don't protect the heart from jumping over the barricades and craving that which the outward standards were built to keep out! So let's not toss away all the external protections, but how about going for the bull's eye, the heart itself?
Psalm 51 doesn't say that God desires conformity to a set of external standards by which others will be impressed and notice our high level of spiritual maturity. Nor does it direct us to avoid particular fashions, hair styles, or all forms of popular music. It says that He desires honesty and truth in the innermost parts of our being. David asked for God to create in him a clean heart and a right spirit!
Again the outward restrictions can serve as a guard from outward influences, but do not guarantee pure motives. The internals are only transformed by the work of the Holy Spirit, the supernatural effects of God alone. Psalm 15 and 24 both identify the one who will dwell with God. It is the one whose heart is pure and who speaks the truth in his heart, not the one who made the most commitments, memorized the most Scripture passages, or even struggled to keep a good attitude.
Cleaning the outside is important but represents a human work which produces pride, inferiority and frustration. Letting God grow the work he began in the heart at salvation is the only lasting motive-changer. Externals don't affect desires. Only God can produce in us desires that please Him. All the God-infused desires we need are present in the believer. When the internal desires are fed and recognized, the externals will take care of themselves. Then the outside will accurately reflect the inside.