Late Game Tactics with Protoss

Late game, if you have the resources, Protoss can be downright
fearsome. You should have plenty of Templars to support your forces with
Psi Storm (arguably the best spell in the game). Keep using the Shuttles
to drop them in places where they're not wanted and Psi Storm masses of
idle enemy units. Even if you're heavily outnumbered, a couple of well
placed Psi Storms evens the battlefield quickly if you can protect your
Templars. The key to doing that is to press the attack as much as
possible. The more of an offensive stance you take, the less chance Zerg
or Terran players can "assassinate" your Templars with Queens or Sci
Vessels, because they're too busy managing other things. Also, a glut of
Templars can be turned into powerful Archons. A few of them can greatly
discourage swarms of Mutalisks, but just don't rely on them too much
against a Terran player with Science Vessels.

Arbiters should play a role in your attack if you can spare the gas for
them. Their cloaking ability can be very useful. Recall should also be used
in the back side of enemy bases whenever possible. Recalling a group of
Reavers and Archons will wreak immediate havoc on the opponent. Also
research Stasis for use against masses of Guardians, Mutalisks or
BattleCruisers. Don't hesitate to use Hallucinate to make copies of your
Arbiters for protection.

By the late game you should have several Scouts to take down dangerous
enemy air units. If you should choose to make Carriers then you definitely
need a fleet of Scouts to protect them. Scouts are very useful for taking
down masses of Guardians, Mutalisks, or a few BattleCruisers. Hunt
down any stray Queens, Science Vessels or Overlords you see around
the map.

Keep up pressure with the sturdy Zealot on the ground, supported by
Reavers as in the mid game, and use a combination of Scouts and
Templars to stave off attack from the skies. Observers should be
scattered all around the map during the late game, so you're never without
a detector nearby to spot cloaked units.

Next: Defensive Schemes