Micromanagement:
I just don't want to talk about this obvious one, but I have to.
One word: Controlgroups. Ctrl + [Number] and there you go. The best way how to do it you have to find on yourself.

Another way of controlling your creeps efficiently is the Subgroup Control Key.
Credits for this go to Eumellein.
Holding down the Ctrl key while issuing an order only gives the order to the highlighted subgroup. This feature is very handy for using concentrated firepower and moving Heroes into and out of battle without breaking your selection. This key must be enabled in the options menu: Options --> Gameplay --> Subgroup Order modifier key.

In easier words: Press the Ctrl button and give a command (for example attacking a unit). Only the highlighted subgroup (for example your hero) will follow your command. In order to use this feature, you have to activate it in the Gameplay menu.