There's a tutorial by the artist and animator Aaron Blaise about creating a squash and stretch effect for character animation. This is my very rough attempt at creating one for a character I made up. I made this in Photohshop and recorded it to see how long it would take me and to look back over any mistakes I made.
The tutorial can be found here: https://www.youtube.com/watch?v=6Vsp2Z1IIy4
In the tutorial Aaron Blaise is using TVPaint as his animation software, but seeing that I don't have that and I don't really like using Flash, I went for Photoshop and created this very short piece in there.
Over all this took me around forty minutes to complete, and the video editing took a further thirty minutes to do.