I've been looking for a solution to prevent my props from coming off. When I initially built my copter I tested it and had two props fly off. It was just a ground test for pitch, yaw etc. It surprised me. I tightened the nuts and after a few more tests I had the same thing happen. My solution (I'm open to correct methods) was to use plumber's tape on the thread and Scotch 5lb mounting tape above the nut. Believe it or not, the mounting tape absorbs the vibration that causes the nut to shake loose. I've had several flights without issues. After every two flights I check to see if the nuts have loosened and haven't found one in danger of coming off (loose). Any correct methods for securing a nut would be greatly appreciated. I've seen enough videos of multicopters crashing from props that came loose...not to mention those that break.
Nylock is the answer. Forget what I said above. Nylocks and a washer will keep your props on.