On crash courses, use the Right Analog-stick to point up on the countdown. On each second, rotate it in a circle. On the last second, rotate it around twice to get an extra shot of boost.

Get three takedowns so that you have a good amount of boost in your boost bar. Drive in the oncoming lane with your boost. Your boost will not go down and become infinite until you crash or go out of the oncoming traffic. Note: This works even better in Burning Lap, Preview Lap, and Special Event modes. You will actually start gaining boost when this is done in those events.

In Crash mode, press L1 and R1 repeatedly during the countdown. Sometimes you will start with about five seconds of boost.