I'll Shoot, You Drive
Teamwork is key to managing most vehicles effectively and knowing where your strengths lay is vital to dishing out the most damage while using them. If you're a shit driver, don't hop behind the wheel. If you can't aim, don't hog the gun. Do what you do best, and let others do the same. Driving may not be as glamorous, and shooting may not put you in control, but being good at your job will help ensure your team's victory.
If you're the one steering, make sure the vehicle never stops. A single charged shot from a Plasma Pistol is all that's needed to make you a sitting duck while your Warthog, Scorpion, or Ghost is cooling down from the attack. If you're behind the cannon, don't neglect what's in front of you; very often gunners focus on killing the targets they've passed and miss the guy aiming a Rocket Launcher dead ahead of them.