Hint: Easy wins:

When racing for magazine covers, unique performance or visual parts, it helps if you have both NOS bottles filled. This can let you scab that tenth of a second that you have always missed. It also helps, especially in the more difficult and longer stages, with small time limits, to immediately activate the GPS as soon as you have accepted the challenge.

Tap [Nitrous] to go faster and conserve NOS. Always put your NOS at little more on power than duration when in dyno testing and tweaking your car. This will make it easier to win races.