How do you think of a unique idea for a pose or something for your characters to be doing in a picture to keep them from looking so static and inactive?
Do your characters have a story? Have them doing something. Most people prefer to look a a picture that is a complete scene/has an emotion/story behind it rather than just a character design.
I like to think of it less like posing and more like acting. If you have them do something and feeling something a pose isn't far away (a character digs for gold, a character orders food, a character fights space mutants while having a leg injury, etc). My problem is with "modeling" poses, the kind used just for the sake of pretty and showing off the design. I have no idea how you go about that besides gathering a ton of references and inspiration.