the "best" option for smooth slow motion is to shoot higher fps. So you can't get ideal shots with that camera
e.g. if you had 60p footage on a 30p timeline, it would be 1/2 speed
with after effects you can use pixel motion (generate new frames from interpolating) , but the side effect is artifacts, worse in some types of footage than others, and it's very slow to render. avisynth has some options that are similar in quality, but many times faster to render
The other options are a) duplicating frames, or b) blending frames, both which are usually worse and more choppy, but it also depends on the "look" you are going for.