1. Copy several car sketches or renderings you like and that inspire you (remember not to try to pass the designs as your own, and always credit the original artist). Don't worry about not having a unique style - you will inevitably have a unique style no matter how much work you copy . Make sure you're copying professional work which covers a wide variety of design languages to cover a broad range of techniques. Mastering a broad range of techniques will eventually give a feeling of greater individuality to your work.
2. Apply the different styles to your own designs. This might not produce perfect results at first; perseverance is required. Don't just settle for the one you are able to do from the outset; keep trying to master all the styles you like; they will all come in useful. Gradually blend the other styles with the one you are most comfortable with.
3. Try to see your sketches and renderings as the same illustration at different stages. Having separate sketching styles and rendering styles may not help your workflow in the long run. Each sketch should be seen as potentially working towards a rendering and a finished design. If you sketch highly distorted thumbnails you might feel like you're starting from scratch when you come to making a more realistic rendering.
4. Steer away from defending criticism of your work with the reasoning "but that's my style". It is possible to have a style that only works in your eyes. Seek to improve your style beyond reasonable criticism but learn to differentiate constructive criticism from plain negative remarks, especially if you are designing cars as it is so competitive.
5. Recognise the shortcomings in your style and tackle them. If you mainly draw sharp edged, aggressive designs you may not have the tools required to render soft organic forms. Go back to stages 1 and 2 if this is the case and copy some work that achieves what you are not currently able to do. You might realise you have forgotten to blend in some of the styles you originally chose.
6. Is the visual impact of your flatwork reliant upon one ingredient? If your render doesn't leap off the page unless it is multicoloured or full of highlights and lighting effects, try to create the same impact in other ways such as composition or viewpoint. Then when you add effects later they will be much more powerful. Maybe your car sketches only look funky because of huge wheels and no ground clearance; draw forms without wheels; spaceships, boats; anything that will fill in the blanks.
7. Observe reality and apply it to your work. No matter how stylised your technique is, the aim is to depict a real life product so keep an eye on how real surfaces appear and imagine how you would render them. These thoughts will stay with you when you are working and will translate into your style. You should also be able to communicate to non-designers so don't forget to develop that ability. You might even notice something striking on a real car that most people don't include on their renderings; if you can figure out a way to do this your work will stand out.
8. Appreciate the value of subtlety. Truly outstanding images will have a depth of accuracy, subtlety and detail that you might not appreciate at first glance.
9. Introduce aspects of artwork that you like from outside design. If you succeed in producing a convincing rendering with an unusual feel it will seem more impressive and give an air of greater originality to your design.
10. Keep practicing regularly, every day if you can and whenever you feel some inspiration. If you work for fun rather than only on defined briefs you will improve rapidly. Aim to produce something really good every few sketches and don't worry if each sketch is not a masterpiece because they all help you improve.
Being open to developing your style can really boost your progress. Relax and work with confidence knowing that if something isn't working you don't have to show it to anyone and that you can try many times to get it right. If you get lost and are unhappy with everything you're doing go back to copying professional work until it starts to make sense again. Once you are producing work comfortably in a certain way start to absorb styles that you like from others' work and you will find your designs will become more sophisticated along with your style.
I was once told by a tutor in passing "you have a very exaggerated style". I was pretty sure I hadn't been born with an exaggerated style; if anything I had picked it up. I wasn't happy to be pigeonholed like that so I went back and copied more realistic work. You will probably find people will be keen to tell you what your style is so try to make that a difficult thing to do and you will be developing a style to impress.