Pick off stragglers
If the Human team is dumb enough to leave behind a player, rip them a new one! You'll cause the team to rush back to their friend's aid, which can lead them into a trap set up with your fellow teammates, and even if they don't fall into a trap, it still hurts the team quite a bit. Any type of Infected can pick off the stragglers effectively, and it's always highly suggested, as it's easy points and easy damage. Conversely, picking off those that run ahead is great too. Any that wander from the group are fair game to you, so have at them!

Don't be afraid to die
You are infinitely more fragile than your Human counterparts, but unlike them, you can respawn. Granted, respawn is between 10 and 30 seconds in most cases, but it's better than no respawn through the entire level! Don't be afraid to press the assault, freak out the survivors, and just do some damage in general, as you will always come back as another Infected. This is especially true during the finale, where players on the Infected side have no spawn wait times! Don't throw away your life meaninglessly, but don't hold back for fear of death either.

Learn the four Infected classes
There are four kinds of Infected you get to play in Versus: Boomer, Hunter, Smoker, and Tank. The Tank is the only one that doesn't appear in normal play, as it requires the AI Director to triggers its spawn before a player can start tearing stuff up. Keep in mind that the longer the survivors take, the higher a chance of a tank appearing throughout the level. So delay them as much as possible! Here's some specific strategies for each class:

• The Hunter is great at startling the survivors as well as pinning down moving players, both with his presence and with his actual attack. To pull off a pounce, you must crouch for a short period of time, and then you can leap long distances with a loud scream by pressing the left mouse button. As long as crouch is still pressed, you didn't hit a player, and you haven't been bashed, you can still leap after a leap, bounding across the landscape. Upon pouncing a player, rapidly alternate between the left mouse button and right mouse button to rip them to shreds.
• There is no better way to separate the survivors than by using the Smoker. The Smoker must wait until his reticle is red, and then he can press the left mouse button to attach to a survivor. If the survivor doesn't kill the Smoker within the first two seconds, they will be dragged towards the Smoker until they are dangled off an edge or next to the Smoker. Dangling automatically deals damage, but if they are next to the Smoker, the he has to attack with the right mouse button to deal good damage. The same weaknesses apply during the main game as well, and once you've latched, if the tongue is knocked off, it takes quite a bit to recharge again. Dragging survivors away from the group is a great way to make them panic as well as divide them into more manageable chunks.
• The Boomer is, arguably, the hardest class to play. He dies after a single shot, has virtually no way of damaging the survivors directly, and is about as sneaky as a shotgun. However, they are also the most satisfying class, as the Boomer has the most potential to ruin the survivors. Just like in the regular game, Boomers can vomit on survivors to summon hordes of zombies. The kicker is that their vomit is a lot longer range than the AI would have you believe, and continues for a while, letting a Boomer soak an entire team if he's good. They are best played alongside a Smoker or Hunter, that way the players can be soaked with minimal effort and risk.
• The Tank is... well, the Tank. He's big and he doesn't screw around. His regular attack, triggered with left mouse button, looks like a hug. However, it flings any objects that aren't tied down around. Including players. There's nothing quite as satisfying as launching an entire team off of the helipad in No Mercy using your awesome hugs. The second attack lets you rip a chunk out of the landscape and throw it at wherever your cursor is, dealing massive damage. There's not much to it. On top of these attacks, if the Tank player doesn't engage the survivors in combat for a while, the Tank will go ballistic and be turned over to AI control until he calms down a little. Overall, a very fun class to play, but a rare treat.