One option is to use baking soda because of its natural whitening properties, it also has a mild abrasive that scrubs surfaces of the teeth, it is reported that toothpaste that contains baking soda are more effective is teeth whitening than the rest, another option is the use of apple cider vinegar which has a bleaching effect, but caution is applied here as the acetic acid in it has the ability to erode the enamel. For me my best option will be baking soda.