Have you ever noticed how the person in or near last place always gets the best items, while the person in first seems to get screwed and gets useless items? Its a handicap. It gives players in last the chance they need to catch up. All you have to do is use this to your advantage.
Stay in the back of the pack for the first two laps. Not too far behind, just far enough behind that you get all the sweet items while your opponents don't. Then on the last lap use one of the killer items you've saved to pass them (or knock them on their ass) and win the race. Works almost every time.
Let's just say it works well enough that this game lost its fun for me and my friends shortly after mastering this tactic.