A strong second kick in butterfly is one of the key characteristics of a good butterfly. In the latest issue of the USMS Streamlines Newsletter, there is a link to a nice article explaining a drill to improve this aspect of your stroke. A video demonstrating the drill was recently uploaded to the USMS YouTube channel.
The GoSwim website has many butterfly drills that may strike your fancy. For decades, my favorite butterfly drill has been Extra Kick butterfly drill, which I find very helpful to developing a good sense of timing and rhythm in the stroke.