Always be on the lookout for green-highlighted objects, including cars, fire hydrants and bodies. Tapping them twice creates a diversion that automatically draws walkers within the area, albeit temporarily. Use this opportunity to get the jump on your enemies and pick them off.
Using a melee weapon (such as a nightstick) over a gun you conserve ammunition which is in limited quantities.
Hitting a walker with a baseball bat creates much less noise, and won't attract a horde of zombies. This way on tougher difficulties the silent kill will save your life.
It's in your best interest to go in with a team of four as added muscle and firepower always come in handy.
When making a team keep in mind each person has individual special moves and team boosts.
Always travel as a group (move in pack not individual characters). This prevents you from leaving someone exposed to walkers and it also saves precious seconds, especially if your goal is to beat the level completion time.
Manipulate The Camera so you can track your team members. Adjust the camera and pinch-to-zoom, both of which give you a better view.
After completing a mission, you can spend points to upgrade the following team attributes: damage, health, ammo capacity and adrenaline (this fuels special attacks). Not a requirement to beat the game, but it'll help tremendously.