Rule # 1 is always that a zergling rush usually does not work UNLESS your against a newbie who is slow. Zergling rushes usually fail and waste time and resources in the early stageses and usually cause you an early defeat. What I like to do is usually get drones and resources fast at the start then get a spawning pool so you can get sunken colonies as defense. Then get about 24 hydras or more then build a second hatchery for more breeding. This ensures you adequate defense as you form a second hatchery. If playing against terran opponent, sometimes a good swarm of guardians and mutalisks is the best attack. Never use Guardians against protoss opponents because they can take out as many as 12 at a time if they have a lot of templars. One more thing I've learned is to never send overlord out scouting because if caught and destroyed you lose a farm. And sometimes waiting for an overlord to be hatched can cost you the game. When playing against protoss the best offense is hydras with some lurkers. No body wants to face a swarm of hydras with all upgrades.