Press R to hop while turning or when making a turn. While you are sliding, or immediately beforehand, keep pressing Left then Right (or vice versa) to acquire a blue flame behind your kart. Release it to get a slight boost. If you keep pressing Left then Right (or vice versa) some more while you have the blue flame, you will get a red/orange flame for an even better boost. You may or may not notice the blue flame when getting the red/orange flame.