Principles of moral conduct For denourishing restlessness and remorse, code of discipline and principles of moral conduct are useful. http://www.accesstoinsight.org/lib/authors/nyanaponika/wheel026.html#rest 
But what are exactly these principles of moral conduct and code of discipline for laymen? 

There are so many codes of discipline(five precepts, eight precepts) and principles of moral conduct. I am somewhat clear about the codes of discipline, but what are these principles of moral conduct? (some principles in my mind are ten skillful actions.http://www.accesstoinsight.org/tipitaka/an/an10/an10.176.than.html)
You have correctly identified principles (IMO) of moral conduct in your OP,  @Dewmini  Gunasekera. For the benefit of others may I list them? While the five precepts are usually stated in negative terms they have their positive counterparts. They are the principles of non-injury and loving-kindness, the principles of honesty, the principles of sexual propriety, the principles of truthfulness and the principles of sobriety that a Lay Follower is to follow.

Some Buddhists observe the Eight Precepts once a month. These three additional precepts are:  Abstaining from eating after mid-day; Abstaining from dancing, singing, music and other forms of entertainment; and  Abstaining from garlands, scents, cosmetics and other kinds of adornment.

For those who are desire a greater degree of withdrawal from lay life, and are prepared to make a full-time commitment, there are two more precepts. They are Abstaining from luxurious beds, and Abstaining from accepting money. For lay Buddhists the keeping of the five basic moral precepts of is adequate but is in itself quite demanding.  

One of the fundamental Buddhist principles of moral thought and action is kamma. Kamma is the Law of Moral Causation. Kamma, literally, means action; but, in its ultimate sense, it means the meritorious and demeritorious volition (Kusala Akusala Cetana). It is the principle of Kamma that prompts a person to refrain from evil, do good and be good without being frightened of any punishment or tempted by any reward. For Buddhism the relevant kind of action is volitional action, deeds expressive of morally determinate volition, since it is volition that gives the action ethical significance. Thus the Buddha expressly identifies action with volition. In a discourse on the analysis of kamma he says: "Monks, it is volition that I call action (kamma). Having willed, one performs an action through body, speech, or mind."

The Majjima Nikaya Sutta Number 24, Rathavinãta Sutta, shows us the importance of the Purification of Morality (Virtuous conduct). Moral discipline is the foundation for concentration, concentration the foundation for wisdom.  The Pali word for "moral discipline," is Sila. The observance of sila leads to harmony at several levels — social, psychological, kammic, and contemplative. At the social level the principles of sila help to establish harmonious interpersonal relations.

The ultimate moral code taught by the Buddha is beyond the changing conditions of the world. For example a disciple is a person who has submitted to the Blessed One and His Dhamma. To yield or surrender (oneself) to the will or authority of Buddha and Dhamma requires real discrimination—the capacity to recognize the necessity of completely opening oneself and letting go. The more one surrenders, the more accountable one will become for ones own choices, ones own practice, and for the terms of one’s engagement in this path. If you find a person who understands submission to authority and you’ll see a person who is humble, full of love, unselfish, accountable, and personally responsible. Find a person who does not understand submission to authority and you’ll see a person who is prideful, full of criticism, selfish, self ruled, and spiritually irresponsible. Such mindful surrender is an opening to a deeper dimension of truth.