How do I effectively use cloaked units?

One of the biggest threats to an unwary player are cloaked units as, without any detection, they can ravage entire armies and bases without any retaliation.

My main problem in their use is that players that have ever played a Starcraft game and AI know to build detection of some sort. Because of that I find cloaked units hard to deploy effectively without getting slaughtered. This is especially true when I try to use dark templar since they are melee units.

So my question is, how can I fully utilize cloaked units (specifically ghosts, dark templar, and banshees) in SC2 multiplayer?

Cloak in Starcraft 2 is in general used in two circumstances:

Rushes

Cloaked units is often rushed to early on for a chance to do damage. Dark Templar and Cloaked Banshee rushes are exemplary. The idea of a rush is to get them out as soon as possible, and hit your opponent before have preparations against sufficient mobile detectors.

Sneaky Harassments

The other use for cloaked units is harassments, mainly workerline harassments. When an opponent has multiple bases, it is quite possible that one of his workerlines is not well defended. A cloaked units can easily sneak across the map, and possibly attack without fear of reprisal for a while before a detector is brought in.

In general, Cloaked units should not be gathered en masses, and should strike separate locations to overload opposing detectors.

Many things in Starcraft come down to timing and awareness.

It sounds like a no-brainer that an unaware enemy will be caught off-guard by cloaked units. But the heart of the concept is that your opponent cannot prepare for everything.

But if you combine it with timing, it is even more powerful. Many strategies in starcraft rely on timing. A 6-pool is a very cheap strategy to inflict early harrassment on an unaware player. It can be retaliated, but it requires awareness. The same goes for a terran-drop. Or for a cannon rush.

So, you might say that you can simply scout everywhere and prepare to build all necessary counter units. But that is not possible in a fast-paced starcraft game. You need to evaluate the cost of every unit.

Cloak-Scenario: Building a detection unit/building is expensive. A rocket turret or science vessel is something that comes very late in the techtree. And energy of your orbital command is much better used in mules.

And even if the terran decides to build a turret, the turret can only be in one place. If he builds a science vessel, it can only be in one place, and even the scan will only persist for a few seconds. So in addition to having the need to be aware of your strategy and to be able to have the correct counter unit out at that very time, your opponent has not beaten your strategy yet. You can simply dodge his detection, split your forces and therefore force him to commit even more resources to fend off your strategy. And since you only need very few dark templars for that, you can freely build up an entirely different army to transition to the next timing move.

The same applies for Protos: Canons might not be part of most players build plan and observers are fairly late in the tech-tree.

And for Zerg: They will need to upgrade their Overlords, after which they will be able to detect you, but only in one place at a time and move slowly. Sporecrawlers might also not be present yet.

So in short: inflict (timing) pressure on your opponent by perfecting your build order. And things will get alot easier.