Well really, all you need to do is just animate more frames, closer together. Maybe throw in a little double and triple framing.

Quote from Cronos

Maybe throw in a little double and triple framing.

Although that's iffy if you're using a frame rate of less than about 22.

Tweens might be good enough if you don't mind it looking a little weird.

Animate in between frames if you don't want to tween. Just animate MANY MANY frames that represent a movement, that way the move seems to pass more slowly and in term creates a 'slow motion' effect.

thank Mang0 for this:
http://mango1992.deviantart.com/art/Slow-Motion-Tutorial-203359317?q=gallery%3Amango1992%2F25649289&qo=9

Quote from Cronos

Well really, all you need to do is just animate more frames, closer together. Maybe throw in a little double and triple framing.

triple framing is better for 24 fps