You need to practice all strokes to improve your freestyle.
"The off-season is the perfect time to learn better technique in butterfly, backstroke and breastroke. Aside from the body-balance benefit of working different muscle groups in the water, each of the other strokes has elements that can be applied to better freestyle. The catch motion in butterfly is similar to the freestyle catch, and increased butterfly proficiency can lead to better awareness of how it feels to hold more water during the freestyle catch. Similarly, the breaststroke catch can be thought of as a freestyle drill, with increased water pressure and pitch awareness on the hands. Finally backstroke works the opposing muscle groups and can help activate the lats through effective elbow position, just like in freestyle. Backstroke can also help with body position and balance, as it is a long-axis stroke, as is freestyle."
Taken from Coach Joe Filliol and Triathlete Magazine