Hint: Easy NOS:

Get on the highway and drive in reverse. After a few seconds you should begin racking up points on the right side of your screen. These points will give you extra NOS. Note: Crashing into traffic at high speeds will reduce your NOS.

Go to any shop. Enter the shop, then leave it. You will have a full tank of NOS.

Get on a road and keep doing 360 degree turns using the handbrake. This will give you 400 points and will add to your NOS. You can repeat this another two times.

By driving your car backwards (without hitting any cars or walls) you can fully refuel both of your NOS tanks.

Enter the freeway and keep driving in reverse until the score reaches about 3000.

You can recover NOS by doing one of the following things.

Sprayshow: Purge your nitrous
Traffic Hit: Penalty from hitting traffic.
Lead Lap: Lead a lap in the race.
Clean Section: Make it through an entire race section without hitting anything.
Clean Lap: Make it through an entire lap without hitting anything.
Shortcut: Take a shortcut.
Avoid Accident: Almost get in a wreck with another car.
360: Perform a 360 or a doughnut while driving.
Best Lap: Score a best lap during a race.
Head Start: Get off the starting line the quickest.
J Turn: Drive in reverse and swing the car around to drive away forwards.
Burnout: Spin the tires.
Leap Frog: Jump your car over a traffic accident.
Hydro Bounce: Bounce your car using level 3 hydraulics.