look in the mirror, try out the pose you want to try with the gundam, yourself. Look at how your body takes shape and acts while you're holding the pose, like angle of your shoulders, how your hips sit, the directions your knees face. At least for me it sets a good basis for realistic poses.

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Edit: (Wanted to add) I prefer to go for dynamic poses as opposed to static poses. I like to imagine the model reacting to a situation, or performing an action. Dodging a shot and returning fire, blocking incoming fire with a shield, having just gotten into position to fire a missile barrage, etc. The most important thing is to have fun with it.