Creep Stacking

Creep Stacking is the art of drawing neutral creeps away from their camps just before they would "respawn" (see: Neutral Creeps). Every time neutrals would respawn, the game checks the area in which they spawn for any units. If a unit is found, no neutrals will spawn. By pulling the neutrals away from their spawn point at the right time, the game will "respawn" the neutrals, as there are no units in the area, allowing a player to double his gold income from a single camp. It is advised to stack a creep camp before pulling it.

How to stack
Stacking creeps is all about timing. Most camps can be stacked by getting the neutral camp's aggro (by walking close to them while they are awake, or attacking them) around the 51-53 second mark. Once the creeps are following the hero, you should move your hero as far away from the camp as possible (bringing the creeps with you. Following a straight path will ensure that the creeps do not lose sight of you and turn around. If done correctly, the creeps should be away from the camp when the timer hits :00 (a new minute) and new creeps would have spawned.
 * Note: Neutral creeps only follow you for about 7 seconds, after that they head back to their camp.

The above method can be done multiple times, but timing might start to differ (as you have different creeps running at different speeds). A camp can be stacked indefinitely, as long as you get all the creeps out of the camp by the time the creeps would respawn.

A conprehensive guide to creep stacking can be found here.

Side note: The area in which the game checks for units differs for each camp. It is not necessarily exactly in the area the creeps are, but sometimes a little further.