Flutter kick is the motor that drives a swimmer's freestyle, but outside of moving the legs up and down, swimmers and (sadly) coaches often ignore the leg component of the stroke. A little attention to detail and some specific mobility and strengthening work will make you a better swimmer over all.
Stretch out your hip flexors. Many swimmers get caught in focusing on the down beat of the flutter kick - when the foot is moving towards the bottom of the pool. As a result, the hip flexors (in the front of the hip) get very tight and prevent end range hip extension. By working on a more balanced kick, and moving water with the front AND the backs of the feet and legs, you will have more hip drive and better propulsion through the water.
Stretch your hamstrings. Tight hamstrings lead to too much knee bend in your kick, which often follows tight hip flexors, and prevents full hip extension and thus, creates imbalance in the kick. If you kick on your side (either on the surface or underwater), it will become evident if your kick is imbalanced. If you're running into the lane line and find it difficult to go straight down the center of the lane, it's time to address a kick imbalance.
Work on your ankle mobility. Ankle circles in each direction and improving your toe point (plantarflexion) will allow for a whip-like action at the end of your kick - think of the snap of Indiana Jones' bullwhip.
Kick more. A lot more. But kick mindfully - don't just go through the motions and use the opportunity to catch up with your teammates (I know, we LOVE a social kick). Feel water move over the top and bottom of your foot, and think about using your full hip range of motion. Vertical kicking is a great tool for improving balance in your kick, as is changing up kicking with a board versus kicking in streamline and playing with your body position while kicking (underwater versus at the surface, kicking on your side, front or back).
If you feel like you have pain or difficulty achieving any of the above mentioned exercises, it may be time to get evaluated at Streamline Physical Therapy!