Best slow mo comes from command line tools on Linux. Said no one ever. But that's what I typically use because it's quick and easy. [1]
An even easier method is to use VLC (free for Windows and Mac, too), which allows one to reduce play back speed, then record that. This method can produce ok results if the source video is, say, 60 fps or better, and you're looking to play back at 24 fps. Otherwise need a video editor that can fake it in post processing. [2]
[1]
http://giantdorks.org/alain/set-fram...tion-playback/
[2]
http://www.videomaker.com/video/watc...and-clip-speed