I used a program called alchemy which lets you draw random shapes with a black pen. When drawing shapes, I looked for a potential vehicle shape which I could bring into photoshop and clean up by removing and adding parts but keeping it in black.
The next technique is using a custom paint brush and changing the different settings such as scatter and rotation to create a unique shape or pattern. You can then remove and add parts similar to the first technique to create a more visible shape.
The final technique is using an image and using the lasso tool to get a part of that object. I then mirrired the part of the object and used the brush to fill in th gaps. This is my least favorate way as I dont think it looks as good as the other techniques.
Daniel Porter 