Like Mel said, kick on your side. I like to do 10 kicks on one side, then stroke over to the other side and do 10 kicks on that side. I do most of my kicking with fins. Kicking on your side really helps you with stretching out your body and working on body position, etc... You don't need a very strong kick. Just kick enough to keep your feet from sinking. Your core is what propels you, not your kick. If you kick too much, then your legs are tired.