Some tips that help me:
Focus your workers & sellers in one single standard limited car until the end. And repeat it all the time.
Using souls, always choose the most expensive option.
Raise your STAT "LIMITED MAX COUNT" (280 souls) up to 2500.
Riase your SELLING SPEED 3 x PRODUCTION SPEED
Raise your SELLING SPEED 3 x MAX SELL PRICE
lvl 223, over 236 million money....
2x RailGun 200 Damage, 50 speed, Max range
4x RailGun 100 Damage, 20 speed
16x standard Inverter Beam
since lvl 130 without modifications
http://img837.imageshack.us/img837/6791/laserdefense.png
 "Idle Car Manager"){kind=icon}
 "Laser Defence"){kind=icon}