var formKeys = {
    'recipient_firstname': false,
    'recipient_lastname': false
};

function clearField(obj)
{
    var key = obj.name;

    if (formKeys[key])
    {
        //do not clear
    }
    else
    {
        obj.value = '';
        formKeys[key] = true;
        obj.style.color = '#000';
    }    
}