The Shuttle is best used for transporting units that either cannot travel very fast on their own or cannot travel over certain types of terrain. In land warfare, you will usually want to keep the slow moving Reavers and High Templar in Shuttles to quickly get them to the front lines or to execute sneak attacks against enemy bases. In island warfare, Shuttles are absolutely essential for transporting ground units from island to island, whether as part of a "drop" attack or for the creation of expansion bases.
Every player should remember that when using Shuttles, you must always be ready to unload any units being carried by a damaged Shuttle. If a Shuttle is destroyed while transporting units not only is the Shuttle lost, but the units inside will also perish. This can be a crushing blow to a Protoss player. Remember that once a unit is unloaded from a Shuttle, the unit undergoes a cooldown period before being able to attack.
The Gravitic Drive speed upgrade is something that you should get as soon as you begin Shuttle production. The increase in speed can mean the difference between victory and defeat. However, even if you do get the speed upgrade, be sure to escort your Shuttles with Scouts or Corsairs to increase the chance of a successful transport. Enemy units will normally engage the Scouts or Corsairs first, allowing you time to either unload the Shuttle's cargo or make a hasty retreat.
Another tactic to employ that will increase the chances of a successful unloading of ground forces into a combat situation is creating Shuttle decoys with the High Templar Hallucination ability. In most cases, if you keep the real Shuttles behind the decoys, your enemy will not be able to destroy the real Shuttles before you unload your units.
The Shuttle can also serve as a haven for the High Templar and Dark Archon, keeping them safe while their Energy recharges. While inside of a shuttle, a Templar cannot be targeted by the Spawn Broodling or Irradiate ability, either. This can also be useful for the Reaver while queuing up production of Scarabs.
