Starcraft 2 Zerg training series: General

Build more harvesters! Armies can be cool but expanding is everything

Keep your expansions protected with fast moving units.

Spend all your money. If you have 2000 of anything you did something wrong.

Keep your supply up, dont make it too high till you can afford the extra overlords

Play some zerg vs zerg against players and try out your tactics evenly matched.

Never play defence with zerg, get your zerglings fighting, then your mutalisks/hydralisks and if you have to go for end game units.