ALWAYS ALWAYS ALWAYS keep building when you're moving out for battle =]
This is what usually gets everybody from bronze to gold.
Attacking a mass army, and having your opponent barely fend off said attack to only be barraged with another wave of units (ones built at base).
this is true ESPECIALLY when you're zerg. ALWAYS spew on 25 and rmbr to spew in combat as well as morph. do NOT waste any of your larvae from spawning (cap 3 natural larvae).
DO hotkey your buildings , barracks, gateways, hatcheries, ETC.
Now go out there and kick ass anons.