Starcraft 2 Zerg training series: Unit production

Rally your armies.

Select a group of hatcheres, assign them to a shortcut, rally them all to the same place and use that to build scores of armies simultaniously. This will help you get around the chores involved with finding larvae to breed.

Have at least one expansion more than your enemies to keep unit production high. The zerg produce all their units at the hatchery, so these expansions become crucial mid game.

Build an extra hatchery in base, this is called a macrohatchery, the idea is to receive additional larvae to spend all your funds.