How to fly:

Get to a place where you would like to fly.  Park the warthog with the side seat facing away from where you want to go.  Get in the side seat with a sword.  Have someone else get in a banshee and fly right next to you, so that your sword target turns red.  Immediately when this happens, press and hold the right trigger(fire button).  Have the banshee fly to where you want to go, and look across the driver's seat and press x when you're ready.  Holding the trigger longer makes you go farther.  Have fun with it!

Here is a video showing awesome stuff done with flying in Halo 2

 http://files2.mythica.org/halo/darkhelmet/learningtofly.wmv

The Rocket/Sword trick:

Start a game where you start out with a rocket and sword.  Get out your rocket and put ur target on them.  Make sure that you are close enough so that even when you're not zoomed in your target is still red.  Make sure your perfectly aimed on them.  Switch to your sword and immediately press the trigger button.  If you don't press the trigger button fast enough, it won't work.  To get an idea of how fast you must press it, here's how it works.  Your rocket launcher targeting is red, and when your swords targeting is red, you charge at them, right?  Well if you can switch fast enough and hit the trigger button the xbox will believe it is your swords target and you will go FLYING at them as fast as you would when your close enough to charge.  It's so fun!  You can cover about 50 meters in the game in about 2 seconds.