If your child requests something that you know you will give him if he persists in asking for it, then give it at the very first request. Do not let your child force you into giving it through crying or making a fuss.
If the request is one which you do not intend ever to fulfil, due to its detrimental nature, then do not give in, even if the child makes a fuss. In this way you will nurture your children to understand that making a fuss is not going to force you to give in. if you do not take care in this matter, you will spoil the character of your children.