The first way to not hit power lines is to not fly where there are some power lines. I know that sounds like a smarty pants response, but really that's the only safe way to do it. Even experienced pilots get tunnel vision and get closer to obstacles than they think they are. I was watching a combat match today and one guy flew right into the ground because he was so focused on the streamer he was chasing that he didn't realize how low he was. So the solution is to get a flying site that is far enough away from power lines and trees that you don't need to worry about hitting them.