Hint: Perfect shifting is essential for a good run, and because an automatic tranny just won't cut it, you should learn to monitor redline in all gears and shift right there.
Hint: There's a good reason some cars cost more than others. In the GT3 class, for instance, an unmodified Porsche 911 will wax an unmodified Saleen Mustang. Therefore, if you can squeak by in one series without blowing all your dough on a full load of equipment, do it, and save your money for a better car in the next series.
Hint: When purchasing new components, it pays to upgrade slightly in as many categories as possible, rather than going real heavy in just one or two. For example, it's better to sport midrange tires, midrange brakes, and a midrange turbo charger than to go with an expensive high-end turbo and marginal handling.
Hint: Learn the course in advance! Then practice until you know exactly where each turn begins, what its length and radius is, and where it ends. And make use of all that track width--enter corners from the outside, cut in close to the apex, and swing out wide again for maximum speed. Remember, inconsistency coupled with the odd flash of brilliance will get you nowhere fast.