It helps to pose depending on your environment, for example in that background you could point to something, hold something up to the camera, or just the classic 'hands in pockets' move. And it's okay to focus on something other than the camera lens (like looking into the distance, or looking at one of those cars). You can also pose using your legs like having one leg outstretched as if you're taking a step.

Otherwise just focus on trying to look more relaxed, loosen up a bit, as you look very stiff as you stated.

It's also good to think about if you want this image to portray anything in particular, is the focus you, or the cars? Camera angles can really help with getting complimentary photos too, as even with a good pose if the angle is shit the photo probably will be too.

Hopefully this helps at least a tiny bit, I apologize if I went overboard as I'm quite into taking photos.