Don't waste your $. Healthtap physicians have repeatedly addressed this subject with a clear "no" answer to colonics. There is virtually no value to colon flushes, cleansers, enemas, irrigation machines when the technique is accomplished to "detox" or eliminate "accumulated waste." the "no" response also applies to liver detoxes! There are few real health benefits, but sigificant unjustified risk. Don't "cleanse".
Neither. There is no need for either. If constipated buy some MiraLAX (polyethylene glycol) or mag citrate.
Lifestyle. Nothing can prevent colon cancer, but high fiber diet, low fat, minimal alcohol, low red meat, can all be helpful.