Some of the races in san andreas are very hard but this is how to win every race:

1) enter the "pedestrians attack with guns" cheat, it is- X, L1, Up, Square, Down, X, L2, Triangle, Down, R1, L1, L1.

now all of your opponants will get out of thier cars/ motorbikes at the start of the race.

2) (if it is a street race, enter the "no pedestrians, hardly any traffic cheat- X, Down, Up, R2, Down, Triangle, L1, Triangle, Left. to stop lots of peds shooting at you.)

now you'll easily win any race ;)