If you want to complete all the circuit races and the GP races without too much trouble then you should do the following:
Go to your garage or search the city for a motorbike(The fastest one who looks like a professional racebike. Don't know the type) and enter any of the races you would like to win. You get on first place really easy and you should start to run away from the rest really easy en win the race without any problems. This works on every difficulty level and both era's.
Added by: Dante_Vale Nov 16th 2009, ID#7902