I could say that I’m pretty good with Synfig. But, one thing I still do not know how to do is how to make slightly 3D animation. For example, I do not know how to make a character’s hand move forwards towards the viewer. What are some good techniques on this?
The basics are: size, perspective and shadow.
ball.sifz (6.86 KB)
You can make different size with scale. Perspective involve size, drawing and relation with the other objects of the scene. Shadows assist the viewer to follow the object.
How to animate that? Well, I think there is no a magic way to do. Sometimes you only need to work with a couple of scalings. Other times you need to work with the full draw to force a new perspective.
For example, take a look to my last animation:
youtube.com/watch?v=u0lam9VkPe8
Well, there are any calling from Disney’s Studio in my telephone 
, but I think it looks enough “3D”. In that animation, I just only play with the draw to make a better perspective of the movement.
How to practice? Place you in front a mirror and act. Study your movements and try to move the the 2D screen. Another thing is to record you with a video camera while represent the movement that you want to animate.