When you are at the start of a race, tap your opponents to lock them in as targets then shoot them until they explode. You can now take your time in the race and win every time.
Before you race, get a gun that is capable of a drive-by shooting. Go to the starting line. When the race begins, allow your competitors to get in front of you. Stay close to their cars, and keep tapping to shoot their vehicle. The car will eventually begin to burn and explode.
Less police
Tap the map, tap "Services", then tap "Corruption". You can "buy" less badges, and the police will not come after you. It costs $1,500 to get rid of one badge. This may make some missions easier.
Portable shop
Tap the map, tap "Services", then tap "Weapons". Once there you can buy more health, or the pistol, SMG and shotguns. This is useful when you are surrounded by the police and are running low on ammunition. You can also buy cars, but they are mostly at parking locations.