Many cameras offer video at a 60fps frame rate, which I find very useful for smooth slow motion. Here is a sample shot with the Nikon D600 (shot at 720p, 60fps).
After shooting the video, I edited it in Photoshop CS6. I split the video at the point where I wanted the slow motion to start, then split that second half again at the point where I wanted slow motion to end. Then I right-clicked on the middle slow-mo segment to change the playback speed. (I also had to adjust the ends of the middle and last segments to provide a smooth transition from slow-mo to normal speed.)
Although the video segments now have different frame rates, I didn't have to change their individual frame rates to make them consistent. I just rendered the entire video to a specified frame rate, making the frame rate consistent throughout the video.
Nikon D600 Hands-On Review