Hey Dude,
I think if you have any spare cash rolling around in the coming months you could really benefit from DVDs by Harald Belker and Scott Robertson.
They show some excellent technique in developing awesome vehicles. It is important to remember the textures of cars. Spend some time working on layering your different painting/rendering effects to develop contrasting textures which define attractive cars. This will really boost your rendering output and give that WOW factor people immediately look for. Try stepping away from your screen maybe a couple of feet to really see where you may be going wrong with your perspectives. EVERYBODY gets perspectives wrong first time round - you have to play with your linework before you can really convince the viewer that they are looking at a believable concept; the rendering adds the sparkle!
Keep it up bro - it's nice to see people not giving up too easy!
Peace