Arcs

In the natural world things rarely tend to move in a completely linear fashion, and the easiest way to avoid animation that looks like this is to draw in all the series of arcs that the animation follows in. obviously there are some exceptions like this such as the head movement of an owl or a few insects, yet the majority of living organisms tend to move in a slightly circular path due to the structure of their skeletons. You can avoid this technique if you wish to create really cartoon-like animation, but in doing so you have to make sure that a lot of emphasis is highlighted on the key/action frames, which as a result must be very strong and intriguing. However you have to make sure that you are very careful as too much avoidance can result in slaughtering the essence of the action.
Usually these arcs either follow a wave like path or a figure 8. This was the only basis needed for the animator to draw guidelines circular from each of the key frames so that the in-betweens can follow the right path of movement. However this can be somewhat difficult to achieve sometimes ‘it is only as a series of drawings is “rolled” on the pegs that the proper location for drawing becomes evident,’ (F. Thomas and O. Johnston, 1981 p.63) has been the way to explain how to attempt to avoid any taboos in this area. All limbs when they move must arc instead of moving in a straight line as otherwise they will not maintain their length and will appear to grow and shrink. A good tool to use (especially for animals) to get say a realistic walk cycle from a side view, using arcs to plot out the eye position for each stage of a walk can make it flow that little bit better.
The straighter the movement the more power the movement has behind it. This is why once objects with no continuous momentum are fired from a long distance they begin to lose power as gravity begins to pull them to the ground, thus creating an arc. The more complex the object and the larger the amount of separate actions included, the more arcs there will be. The key is to treat each of the elements separately, and adding arcs to each of them as so.