I agree with what Mana said, the more you practice the human form, the more expressive and...uh...flow-y your work becomes. Another helpful hint is to, when comicking, think of the scene in your head as a movie. When you find the 'screenshot' you want, draw that frame. Think about how the actions would start and how they would complete. This will help with making your action/movement panels seem less stiff. Also, exaggerate motions. People talk with their hands, yes? Let your characters talk with their hands, but make their motions larger than life. This is something that onstage actors do in order to make their actions and emotions clear. A person should be able to look at the panel and know 'Oh, that guy's really angry'
or 'She looks really upset'
without reading a single speech bubble. Exaggerate what your characters are doing in order to make it all the more clear what it is that they are doing or what they are talking about. (Though, don't make it so
expressive that if people were to really talk like that, they'd hit each other in the face
) Practice at it. Watch a few movies or onstage performances of plays and see how people convey feeling to even the back row. Read a few books to see how writers describe the progression of action. The one thing you really don't want to do is look at other comics. (Your work will begin to look too much like someone else's and even professionals can be accused of the 'stiff' problem) Learn how to do it without the aid of comics. And practice, a lot.
Thanks, Landsin, for opening a new thread.