Are amalgam fillings harmful?

Yes, but not the way that people are commonly saying they are. There is no reputable scientific evidence that amalgam fillings are toxic and harming people by their inherent "toxicity". Most of the reports of their toxicity are anecdotal and not based on real Science. Amalgam fillings, especially large ones ,are not good for teeth because the are too hard and inflexible. They transmit stressful forces to the teeth and it is common to find cracks in teeth restored with silver fillings. It could be that they allow harmful sress points between the filling and the tooth to cause cracks to form or it could be that tiny cracks may form in the teeth as a result of the dentists drills and the fillings put unkind pressure on these microscopic cracks which cause them to propagate.
Gold fillings are kinder(don't transmit forces as directly to the teeth) since gold is more malleable(it bends more easily than tooth) than silver(amalgam) fillings. Most patients don't want gold teeth and these are expensive as well. In my New York City dental practice most patients are having white composite fillings in their teeth, but the verdict is out on whether teeth filled with white fillings do better over time than those filled with silver. Dentists for the most part think that bonded composite is indeed kinder than amalgam, but only time will tell.

How often should I replace my tooth brush?

It should be replaced when the bristles start to be curved. Instead of being straight, the bristles get bent and don't clean as efficiently. The time interval for brush replacement depends on how often you brush and how hard you apply the brush. Tooth brush companies maintain that you should replace your brush every 3 months,but the time varies from person to person. If the brush looks worn then you should replace it.

Why are cleanings important?

Prophylaxis, cleanings,recall visits(all the same thing) end up reducing the load of harmful bacteria in your mouth. Over time many different types of bacteria colonize and multiply around your teeth . Some of these are harmful and activate your immune system causing inflammation. This inflammation can lead to bone loss and periodontal disease. When teeth are cleaned the tartar and lots of plaque(bacteria colonies) are removed resulting in a cleaner mouth. The relative amounts of harmful bacteria are lessened and your immune system is less stressed. Some of the most harmful bacteria are anaerobic and take a while to form around the teeth, usually requiring a thick film of plaque that has been in place for some time. Frequent cleaning every three months are very effective in preventing even healing periodontal disease , since for a period of one or two months the mouth is disease free. Most Dentists agree that gingiva(gums) look healthiest the week following a cleaning. By having your mouth cleaned frequently on the average it is healthier and more disease free.