When attempting to fly in lucid dreams, it's easy to make the mistake of just putting physical effort into doing it.
Originally, I made that mistake and it would get me a few feet off the ground, before it got too difficult and I fell back to Earth.
The trick is to believe that you will be able to fly. Jump off the ground, thinking "I'll just keep going up." shut out any negative thoughts, and know that you can fly. This method works for me every time, because you're convincing your brain that, despite normally being unable to fly, it is able to in a dream.