Some factors to consider: What frame rate are you animating in, the amount of changes you're making from one frame to the next, and the pacing of your sequence.

12 fps or 15 fps looks different from 24 fps or 30 fps.  You can achieve smoother movements by increasing your FPS, but only if the animation you do per frame is smooth.  Jumpy and inconsistent increments of changes makes 24 fps pointless.  Depending on the sequences, small changes are good, but not all sequences require small incremental changes, such as action sequences and fight scenes.

Some tools to consider: storyboards and x-sheets to planning out your sequence and onion skinning to align your animation.  Onion skinning allows you to see previous frames you took so you can see how much your animation has changed and it helps you create smoother movements.

If you're familiar with post-production softwares such as After Effects, Final Cut Pro X or Premiere Pro, try frame-blending technique. Frame-blending is done by creating virtual frames between two frames; something like "Motion tween" in Flash and it's done by the software.

Here's an example picture:

Frame blending is a short cut to better animation and honestly sometimes makes it look worse. Good animation will always be more respected then animation with frame blending. Also you will become a better animator by depending on your own skills, and not on what frame blending can do for you.

I have to disagree with rioforce on this one...

Personally, I think frame-blending can be a very useful tool in stop-motion. I think that the effect it gives to a sequence of frames almost replicates natural motion-blur without going into the complexities of go-motion animation.

And, several older brickfilms, either out of purpose or limits to encoding and compressing programs of the time, appear to have a somewhat blended-frame effect on their animation. The first time I noticed this was in Nathan Wells' Beast.

In the research that I've since done on that brickfilm, I'm pretty sure that its frame blending is a case of the latter cause. However, I once tried to replicate this in a walk-cycle test I did several years ago - which can be seen here. And, honestly, I like it. Stop motion can sometimes look too jolty, and, this certainly fixes it, even if it does seem to look a bit "mushy," as rio said.

The only big no-no you have to look out for when improving animation is changing frame rates. For example, if you're animating at 12 fps, and want a quicker animation to go by without looking jolty, you could half the length of your frames, essentially animating at 24 fps. - Something similar to this, as done by professional animators, is called animating "on 2's".

However, the same doesn't work the other way around. (Or, at least, will gain you a minor bit of backlash from those forum members that are really particular about frame rates) If you stretched each frame to about 6 fps, you'll end up with largely the same process I used on De Mortem. (A process that I'd been using along with The Login Productions for years, and only recently had been called out on)

Long story short - re-shoot if you must due to choppy-ness. Try and stay at a frame rate above 12, and ALWAYS keep to video standards. (So, 12 fps "on 2's" and 24 fps "on 1's" OR 15 fps "on 2's" and 30 fps "on 1's") Animating at 20 fps or 8 fps will NOT allign with current digital video standards, and some frames will either be dropped or blended automatically - neither of which look pretty.

The only times I've seen frame blending in a brickfilm that really added to the film overall is when it has been used as an effect for flashbacks, fainting, and the like. So I believe that frame blending can be a useful tool, but only when used sparingly, never for an entire video.

And to (try) to answer your question, the first thing I'd do would be to start animating at 12 or (preferably) 15 fps; If you don't at least use one of these framerates, it will be virtually impossible to make smooth brickflims. Second, I would definitely recommend watching FancyPants' tutorial called on easing in and out while animating (Rafael9522 posted the link above in post #6). In this useful tutorial, FancyPants not only shows you how to implement a helpful technique (with a frame by frame demonstration), but also shows some common mistakes to avoid. And finally: practice, practice, practice! It's the tried and true method of improvement. ;)

Good luck!

So I'm getting a lot of mixed vibes about frame blending, so I'm confused if I should try it out or not. But if it is not recommended to use frame blending, I'll do what rioforce said and what others have said as to keep practicing.

Not exactly Frame Blending, but it's an example of why relying on post-production effect doesn't always give you the result you want, is Protectors of the Earth by Joleo.  For the uninitiated, it may look very smooth and life-like movement, but if you look carefully, you can see frame blending--or rather After Effect's Pixel Motion--was used to generate the frames between frames to make it look smoother.  Some parts of Joleo's animation were smooth so it wasn't easy to catch the effect, some parts were choppier and those I notice because it causes distortion in the image.

Watch it here and use the frame-by-frame toggler to see how the effect causes unwanted distortions.

Frame Blending should be used as a last resort or if you know what you're doing with it.  Fast motions, don't use it.

^ This. Although, proper understanding of how things move in the real world, and a bit of "ease-in, ease-out" will save you a lot of headaches in editing.

I would recommend improving your animations first and foremost, as, frame blending should be treated like makeup. Too much is a bad thing, and you can only go so far to make a stuffed pig beautiful... Good animation is good animation. No post effect will make choppy animation smooth. However, frame blending, while obviously a matter of different tastes, isn't completely worthless.