I think those look great, but I can tell right away that you are giving yourself too much time to think and refine. You want to feel real loose and easy in your work afterwards. The prime goal is to capture the gesture, not to make a drawing. I think you should try 30 second intervals even if it means you only end up with stick figures at first. My second, far less important, suggestion would be to use a wider less precise instrument, like a piece of willow charcoal (on the appropriate paper). (if you use willow charcoal on Canson "Mi-Teintes" paper then you can wipe it out with a soft paper towel and use the same piece of paper over and over again, it also allows you the opportunity to make highlights if you wipe it with piece of cotton cloth or use a kneadable eraser) Or use a permanent type of medium, like pen or brush pen, that forces you to just ignore your mistakes and keep working.
Other than those, practical things would be to work at arms length with the piece upright at eye level (ie use an easel or some other means to prop up you work securely) (it also may be beneficial to stand because it helps keep you loose) but this may take more of an outlay than you'd be willing to do at the beginning.
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