This trick is uber fun and my friend (you have to have 2+ people) and I played with it for two hours straight. Go to Coagulation, and make sure warthogs are on. Also make sure Rockets are a starting weapon. And finally you have to be on the same team. One person needs to drive the warthog and the other person has to be in the passenger seat with a rocket launcher. The driver speeds at a hill (the steeper the better). Right before you get to the top the person with the rockets shoots straight down at the warthog. It will send you flying in the air. It takes practice to get your timing right but it's so worth it.

*Note:It's also fun to have the other players get in scorpian tanks and try to blast you while you're in the air. That will send you even higher.