My favorite tip to keep your teeth white inexpensively is to brush your teeth with a little hydrogen peroxide (HP) along with your toothpaste a few times a week. No need to pay for a dentist or buy those $20 hydrogen peroxide strip kits. After brushing a bit to get some foam started with the toothpaste, sip (don't swallow!) only about 1/2 a teaspoon of hydrogen peroxide from a glass you've poured a bit of HP into and swish your teeth with it. Then brush as usual. I don't actually measure the HP; I just "eyeball" it. After brushing and spitting out the toothpaste/HP, I rinse my tongue with water, but not my teeth, so the HP has time to work to whiten the teeth. Then after 10 or 15 minutes, I rinse my whole mouth with water.
This will only work on natural teeth, not caps, crowns, etc. If your mouth gets sore or you develop canker sores, then use less often. Your teeth might be a little "sensitive" after--that's normal. At first, when my teeth were quite yellow, I did this three days in a row. However, for maintenance, I usually only brush with HP once or twice a week.
Good dental care is so important, not only to your smile, but to overall health. A healthy person is the most attractive. So take care of those choppers!
For more information about health issues caused or exacerbated by poor dental health:
From Wrong Diagnosis, symptoms of tooth decay, including comorbid symptoms
Tooth decay and gum disease linked to heart attacks
I highly recommend using a Sonicare, if you can afford it--I found mine on sale for $20, and I usually use rechargeable batteries to power it. Be gentle and thoroughly rinse and air dry the brush head after each use, and you can easily extend the life of the brush head to 9 months, or maybe more. You will save much money in dentist bills because it improves the health of your teeth and gums when used properly.
But you can have healthy gums and white teeth with a regular manual toothbrush. Always floss, no matter what type of toothbrush you use. When you floss, it should be gentle, and feel like a tickle, as you gently work the floss around between your teeth. Then use a soft brush head, to very gently scrub with small, slow gentle circular strokes along the gum line and the surface of the teeth. You only need a small pea-sized bead of toothpaste; using any more than that is just a waste of toothpaste (money).
A friend of mine would only brush once per day (until she basically lost most of her teeth to decay; now in her 60's, she has thousands of dollars worth of fake teeth). If you have to pick a time of day to brush your teeth, then pick the evening before you go to bed. However, you can safely brush two or three times a day as long as you are GENTLE and swish and rinse thoroughly with water.