... three things - an electric toothbrush, toothpaste and Listerine.
It doesn't have to be an expensive toothbrush or toothpaste... the brush I use was on sale for under a tenner and the toothpaste is just ASDAs own brand sensitive one!
You don't have to spend forever brushing either... 5 minutes each time, tops!
Just brush all your teeth using small circles on each tooth, in front and behind as well as sides of any teeth you've lost, then a cap full of Listerine swilled around your mouth until your lips start to tingle, spit it out, dry your mouth et voila!
Repeat whenever you remember and you'll have white teeth before you know it!