Loosen up your work a little, bend the rules a little bit. Try to make your linework clean and confident. Check out Yo’s sketch a day thread.
Additionally I’ve found that its always good to have a few references at hand when sketching. So if I was sketching a car, I would perhaps have a picture of a car and also a couple of sketches of cars that I enjoy from other designers and use them as reference/influence to come up with something of my own…
Just an amateur myself, but thats whats helped me so far.