When enemy air units enter your base have a defiler use Dark Swarm over your Hydralisks. This renders them impervious to enemy air attacks and frees them to shoot the bad guys out of the sky.

 Build Zerglings and Hydralisks initially but remember that Zealots and Firebats can go through Zerglings like a hot knife through butter. Use your Zerglings and Hydralisks to attack Marines and Dragoons.

 Gaurdians are your heavy attack force. A group of 12 will destroy nearly any land-based group as long as they stay at a safe distance and are willing to retreat.

 On larger maps, use Nydus Canals to connect your bases. This allows you to build a much smaller defense force at each base because you can teleport units from one base to another very quickly.

 The Scourge are most effective against enemy capital ships such as Carriers and Battlecruisers. So keep a bunch around for this. Note also that the Scourge are significantly less effective against scouts or wraiths. Don't waste them against these units.

 Try to keep a large perimeter of Sunken Colonies and Spore Colonies around important structures in your base. This way even if you leave your base lightly defended, the colonies can work to eliminate the enemy threat.

 When attacking an enemy base with a strictly Zergling force don't waste time attacking enemy units, instead send your Zerglings to attack enemy structures; a group of Zerglings can bring down a structure quickly.

 The Zerg ability to create masses of a certain type of unit by morphing multiple larvae can quickly shift the tide of battle in your favor, if you choose the units well. Keeping extra hatcheries around is an excellent way to increase your ability to manufacture plenty of new units quickly.

 Use Queens to defend against Wraiths. Have your Queen use it's ensnare ability on a group of cloaked Wraiths, and then put parasites on as many of them as you can (this removes their ability to cloak).

 To take out a Terran player quickly, build lots of Zerglings. Upgrade their speed and attack abilities and then launch them at the enemy in groups of twelve. Speed is important; you don't want the Terrans to build up a large force of Firebats.

 Always use Hydralisks, the Zergs' bread-and-butter unit. It can be effective in both air and ground attack and defense. A Zerg player without Hydralisks is a Zerg player who's probably losing.

Use your Queen's Spawn Broodling ability on Siege Tanks and Goliaths, and use ensnare on cloaked Wraiths and Ghosts. If a cloaked unit is escaping infest it with a parasite so it can't cloak again.

 Use the Plague where Supply Depots are grouped closely together. After a few Plagues the affected depots will proceed into the red and eventually will blow up (unless they are repaired).

 Take advantage of your ability to infest a Terran Command Center. Try to attack one early on to open the door for infestation. Remember, though, that you'll need a queen for this, so don't risk any units until you're ready.

 Keep your Hydralisks and Zerglings burrowed until heavy enemy units come close; then spring the surprise. Keeping several groups of units burrowed in defensive positions is always a good idea.

 In the early going build Hydralisks and more Hydralisks. They are very effective against Zealots, Dragoons, and Scouts.

 Watch out for Observers. Fortunately for the Zerg, the Overlord is a detector unit, and you usually have plenty of Overlords to patrol key areas around your base and resource centers.

 When attacking a Protoss base, always attack the pylons near Gateways and Stargates first. Once you take the pylons out, the buildings lose their power and become useless to Protoss forces. Beware, however, of probes that probably will scoot out to warp in new pylons.

 Build up plenty of defensive structures around your base(s). Zealots can make dogmeat out of a lone sunken colony quickly, so always cross protect your colonies with at least two other units.

 Against Carriers, nothing can beat the Scourge's attack. A group of twelve Scourges can take out a Carrier in a few seconds, so always keep a group of inexpensive Scourge lying in wait.

 If any Arbiters come your way, have your Queens use Ensnare on the units that might be hiding underneath the Arbiter's cloak.