function init_field(field_id,otype){
    var obj=document.getElementById(field_id);
    obj.def=obj.value;
    obj.otype=otype;
    obj.style.color='#999';
    obj.onfocus=function(){
        if(this.value==this.def) this.value='';
        this.style.color='';
        this.type=this.otype;
    }
    obj.onblur=function(){
        if(!this.value){
            this.value=this.def;
            this.type='text';
            this.style.color='#999';
        }
    }
}

init_field('id_username','text'); 
init_field('id_password','password');