The next time you find yourself having difficulty with flying, stop and think about just what your situtation is. You are dreaming your own dream, which means it's all in your head, so you pretty much have control over whatever goes on. Thoughts along that line often help me gain better control in dreams, although it somtimes feels like there is a certain amount of resistance.

You could also try using a flying aid - anything to help yourself believe you can fly, be it a jetpack, broomstick, or a jet-propelled easy chair. Anything that works.