I was asked recently by a colleague how I get my teeth so white and thought after the conversation, 'how have I not written a blogpost on this yet?'. It's so simple but oh so effective. It'll save you a bundle on whitening kits and is much safer and more economical to use than bleach. If you add it to your routine you'll soon see the effects (even if you have wine, coffee or cigarette stains).
All you need is some baking soda, which you probably already have in your kitchen. Sprinkle some on your toothbrush, wet under the tap and brush away. It is more corrosive than toothpaste because it's granular so make sure you're not brushing too hard and if you have sensitive or receding gums try using it in conjunction with your usual toothpaste. If you want a gluten free option, try this baking soda.
Begin using it every day (if you have sensitive teeth or gums I'd recommend 1-3 times a week) and once your teeth are as white as you like, use it as a top-up once a week.