Shifting
Shift the precise moment when the triangle turns green. If done correctly, you will get a perfect shift and a short boost. To time this easier, ignore the triangle and focus at the tachometer which displays your engine's RPM on the right. Wait momentarily when the needle gets to the green zone, then tap to shift gears.
By playing with the sound on, you will also have an additional indicator as to when to shift. This will also be extremely helpful in Blind Races.
Upgrades
Spend the money won from races to upgrade your car. Start with the upgrades that increase its power of your car (for example, Engine or Induction) to get better speed. Next, focus on getting good tires and reducing the weight of your car. Finally, get the Nitrous upgrade if you have a lot of tanks.