The best easy/fast/cheap way to do video slow-mo, is (assuming your "master" project is shot in 24p) is to shoot your specific shots for slow-moing at 60i, and with a faster shutter than usual (say, 180th of a second, or so).
If you shoot it like that, you can slow-mo it down to 40% and it will still look good and intercut with 24p footage very well.
To do GOOD slow motion of existing footage where you did not plan ahead and shoot it with the proper frame rate and shutter speed, then you need special software which interpolates the missing frames. Twixtor is one such program, although there are others.
Fantastic Plastic Entertainment, Inc. fantasticplastic.com