Don't tunnel vision on first place. Sometimes the leader is actually the easiest target because everyone else is shooting at them too. Hang around second or third, let others waste their ammo on each other, then make your move on the final lap.
Save your best weapons. If you get a rocket or something powerful, don't waste it on someone in last place. Wait until you can knock the leader back or defend yourself when you're in front. And always grab boxes even if you have a weapon denying them to others is just as valuable.