What should I do for knees that 'crack' (when I slightly twist), which happens from time to time? No pain or visible symptoms; no previous injuries. 

Don't worry. Joints may pop or crack occasionally, but if there is not any pain or swelling and the cracking doesn't cause any other discomfort or activity limitations, then there is no reason to worry.
Nothing. Cracking is not a symptom of any disease. It may simply signify the release of certain chemicals that are contained in bubbles in the joint fluid. Cracking, despite popular belief, is not bad for the joints and will not cause early arthritis.