As an investor, I guess I need to avoid overtrading. What are the risks of overtrading and how do I avoid that?

A long term portfolio has to be necessarily long term and you can get too aggressive about churning it. Avoid getting in and out of stocks too often. Your core investment portfolio must change only if you perceive a serious change in the fortunes of the company due to some emerging trend that could disrupt the performance of the company. When you overtrade you need to worry about the impact cost, trading costs, regulatory costs as well as taxes. Also you are missing out on natural wealth creation when you jump in and out of stocks. As a smart long term investor you surely wan to minimize each of these costs to the extent possible since these small items can compound over a longer time frame.