From what I recall, a flying creature is immune to being tripped, and generally can't be trampled either. They can only be charged if they are within reach of the creature that is charging. If a flying creature is ten feet above the ground, then they can only be hit by a creature with a reach of ten feet or more.
More or less, things work out as you would expect them to.
One thing to keep in mind is the Fly skill. A creature has to make a Fly check at the end of any turn where it didn't move at least half of its speed; it's DC 10 if you moved up to half of your speed, or DC 15 if you didn't move at all, and failure means you fall out of the sky. You also need to make a Fly check whenever you take any damage while flying, and failure means you drop down ten feet.