Upgrades to your bike are sometimes quite costly, but you can make them less expensive by finding certain parts before you take the plunge. When you look around the map and consider the races you accept, you can preview the potential rewards ahead of time. Choose the missions where there is a chance that you'll spin the wheel and earn discounts, or you'll wind up spending a lot more than you have to.
Restart Races to Save Fuel
When you finish a race and discover that you didn't meet a goal (for example, you might be called upon to complete a certain number of flips and come up short), you'll have to spend more precious fuel if you want to make another attempt. However, there's no such penalty if partway through the race, you give up because it's obvious you're not going to meet your goal. Definitely pay attention to such things, and don't be afraid to restart a race if you're coming up short. It's better than having to wait for more fuel.