No methods exist to get porcelain dentures any whiter than their original color, but you can clean them thoroughly or have your dentist clean them in order to restore them to their initial whiteness. Speak with your dentist for recommended cleaning methods if you need them.
The good news is that porcelain is practically impervious to any staining or coloring from what you eat or drink. Your teeth will darken and stain with certain foods and dark drinks, such as blueberries, coffee and red wine, but your porcelain dentures will remain the same color they were when you received them from your dentist. If you choose to whiten your teeth, you cannot whiten your dentures to match.
Check carefully with your dental professional before whitening your teeth at home or having the procedure done professionally to ensure that your whitened teeth wont make your old dentures look grungy or yellow. It is possible to get new dentures to match newly whitened teeth, but remember that your teeth will darken again over time and the problem will repeat itself. By working with your dentist or other dental professional, you can arrive at a reasonable plan for keeping your entire smile at its whitest.