Being a good scout

As a zerg, you have little to no excuse why you shouldn't be able to see a large majority of the map at any time.  Starting with an air unit and constantly making more as the game progresses lets you see many areas the other races can't reliably see until the mid game.  Tips for overlords;

1.  Hover on the outskirts of your opponents base just out of harms way.  This may seem obvious but so often I see zerg overlords cowering in the corner of the map and not doing any scouting at all!
2.  Run your overlord from a terran base once they barracks goes down.  Unless a tech lab is attached, you can be assured a marine is on the way.  Get to safety before it kills you.
3.  Research overlord speed.  This doesn't need to be a top priority as often running back towards your troops is enough to deter some units from harassing.  But once phoenixs and Valkyries hit the sky, be weary and stay mobile.

With SC2, we also have queens now who can turn creep into a valuable scouting tool.  Creep tumors not only spread the creep, but give the zerg sight over wherever they expand to.  Since creep tumors can spawn other creep tumors.  Getting a fast one down and spreading out can increase troop speed and give sight on the outskirts of your base.  Building your "Creep Highway" is a great scouting tool that costs no minerals or gas so do it often.  I suggest assigning a tumor to a hot key and pressing it every so often to remind yourself.

Scout early and scout often!