@David09 - While I share your enthusiasm for good nutrition, I don't believe that all cancers are preventable. That would imply that anyone with cancer has a bad diet, and I've known people on good diets who had cancer.
As for gallstones, I have read that drinking a lot of water helps to flush them out, but whether drinking a lot of water will always prevent them is another story.
Some medical experts are now warning against drinking the recommended eight glasses of water per day, which used to be gospel for people in the health community. I don't recall why, but now all of a sudden too much water can be bad for you.
I think that you should just eat right, exercise, take your vitamins and do the best you can. In the end, however, there are no guarantees of anything.