The Wingsuit ability in Crackdown 2 is unlocked at Agility Skill Level 5. To use the Wing Suit, press the "Y" Button when at the height of your jump or when falling. To effectively glide, nosedive off a tall building, wait for the blue streams to turn red, the pull back up, when your Agent starts shaking (near the height of glide) nose back down and repeat.
You can also quick tap "Y" so you'll have a little boost during a jump near the ground, allowing you to go further. You can repeat the action several times. It also permits you to be faster when you jump during a rooftop race or a Renegade Orb chase.